Where to begin...
The improvement you mention is nice...I guess, doesn't change the fact that no one is going to get the Tdread for its anti infantry abilities, they take it for the av.
That sounds like an extremely narrow perspective on it's overall utility. It's main attractive-point is certainly it's AV-potential, but making it a comparison to the Brightlance Wraithlord made me almost stop reading. It's not like the Wraithlord has a splash on it's projectile.
The unit itself (TDread) is meant to be an overall-blob punishment with anti-vehicle potential. It's cost is meant to reflect it's performance as well. With the barrage being slightly improved furthermore gives players the accessability to diminish and punish blobs. This works in particular very well with Khorne Havocs if synergized propertly. Just because it can't chase and kill vehicles as good doesn't make it "useless", it's a performance decrease, which I'm standing behind as it was to good.
It's strong against all vehicles? Of course it is it's a damn anti vehicle upgrade, is it no supposed to be? Also how is a dreadnought able to chase vehicles? Come on now...
Again, simplistic and narrow/tunnelvision perspective on the matter. On actual games, you could back up the Tzeentch Dread fairly exceptionally well. Vehicles would get caught off-position, and have little to no means of any escape. Whilst punishing for off-positioning is certainly an aspect of the game. The punishment and reward coming from the TDread was the highlight value of it.
Coming even in team-games, where lascannons and slows are commonground, you would have little problem dealing with any vehicles, regardless of almost how many are fielded with just the synergy between slows, and the chase+damage potency of the TDread. Always why the Plague Marine duration was slowed, because with those two together, they would have extremely strong potency together and more or less diminish the useage of any vehicles completely, making certain strategies completely obselete. It was utterly ridicilous and way to strong. I just don't think people genuially tried to find the neccasary combination of the two roster, but coupled with the rest of a fairly standard Chaos t1/t2 composition, you would have a field day.
But no, it's not a Wraithlord Brightlance comparison even to begin with. They're two distinguishable and different units, and just reading the comparison makes my head hurt, just because they share a trait of chase-AV potential doesn't make them an identical unit.
They may have a better fire rate, but as it is now it's not effective, I'll elaborate on this in a bit, first I would like to mention that the upgrade time thing I mentioned is not incorrect as you say, it was something that was changed in retail, I was just asking why it's still like this, since all lascannons have a 10 second upgrade, the havoc one being stuck in its silly animation that doesn't let room for surprise, or reaction to a vehicle, or mobility for that matter, when other set up teams av can just stand there being a continuous threat because of the sudden upgrade, same applies to the predator, used to upgrade fast, now everything upgrades just as fast, predator still stuck being obvious of what you're going for.
This seems more like an issue with people not branching out. The MoT Havoc is actually one of the strongest AV-unit. If the unit isn't good, that can also translate to people not really incorporating them into their builds really well, or their strategies. If vehicles are an issue, then how does not one of the fastest-hard hitting no-windup lascannons NOT deal with the issue? The issue is mindboggling, esp since Chaos has such good turtle-and cover-up for Infantry/Heavy Infantry come into T2. Turtle-strategies for Chaos T2 is one of their strongest aspects.
Again, I don't understand the lascannon change with build. HWT Lascannon upgrade was unchanged in it's upgrade time, so was the Mark of Tzeentch Havocs, so was the Devastators. The only lascannon change was in T3 for vehicles. But I don't even see why they should be mentioned. You bring up both unit correlations, setup teams, predators and misc. stuff. However initially only the HWT team was the one of the concern that you brought up. Altho I don't see what predator lascannons with their build has anything to do with Mark of Tzeentch Havocs underperforming?
I don't think there's a faction other than grey knights forced to backtrack to their t1 units if they want effective av in t2 if they went for anything other than their set up team, going for raptors or even noise marine (because of how unreliable their blastmaster is) is like begging for a walker in t2.
I dont' see how backtracking is even a bad thing? Several races backtrack to deal with vehicle threats at times. SM, IG, Eldar(Brightlance) and sometimes orks with Beamy Lootas. You could pitch in and throw Havocs in there for good measure, but again- this seems abit bias on the perspective on AV-utility. Grey Knights AV-options are far greater now, taken into consideration that Purifier Justicar has heavy_melee, and that with good support, they're quite lethal against tanks/transports, but should still reconsider engaging walkers, which is a intended counter-product unit.
tldr, chaos lacks av on their standard units like other factions, asm are still useful with their melta, so are stormtroopers or warp spiders, brightlance, venombrood, catachans and all the heroes that have effective av upgrades, the list of units fulfilling multiple roles goes on, ironically chaos from having what was considered the best av in retail is now tied up for worst along with gk.
Well, again. You would have to look into the development and resource-pooling that is essential for these races to deal with a specific target. Chaos goes for an economical more advantage t2 to ensure the field of AV-production unit, whereas some other races can field the neccasary AV-productive from their earlier-stages, but requires more economical funding to ensure that straight-line of counter-production wargear requirement. For an example, Missile Lanchers would be a specialized wargear, making them significantly less versatile in their performance, compared to Mark of Khorne/MoT CSMs which enhances their utility aspect more then a mere-wargear cost.
This is even something that was set-in stone by Relic as Chaos-race fundementalist ground-up design. Chaos is more specialized, whereas Space Marines are more generalist-type of an army counterpart. If you don't specialize your army to synergize with each other, it will fall apart very easily. But once you've filled every shoehole of weakness, they gradually become stronger. However, this also makes unit losses for Chaos in particular quite nasty, and can cause quite a penalty of you lose a key-unit that is designated to counter a specific target. Whereas once the SM army snowballs, they've genuially got some means to deal with multi-targets, just not as strong as a specialized one.
I do however find it quite funny how you compare "standard units" and bring up Warp Spiders, Venombroods, and various T2 units. It's not like Chaos has any of those?
Bloodcrusher//Plague Marines sprungs to mind.
I seriously would reconsider the "worst AV" thing tho, it's SERIOUSLY not THAT bad. Espicially not in the later-stages of the game, where I would even give Chaos AV-options almost the crown-jewel. Hell, I'd say it's far better off as it is now then the retail-version where going vehicle against Chaos would be a complete death sentence for yourself.