Again, I don't know why I have to say it, he did the same thing before. You couldn't just blast a tank with your ult by itself, you had to full combo them to trigger his passive multiple times. "could" being the operative word here, if you weren't constantly thinking about using his passive combo then you were a bad vel'koz player.
In the case of tanks you're right. In the case of squishies you could take several procs of true damage by just throwing your ult at them. You could also fire it at tanks indiscriminately knowing that if all else failed you'd still get a bunch of true damage on them (thus bypassing any MR they had purchase to deal with you) without much thought.
Everything you wrote after this point gives me the strong impression you don't play Vel'koz, and question what you have written down... Why do you assume I max Q EVERY game? I play support, a word you seemingly glossed over, his W was max'd for wave clear, why would you take waveclear in lane with your ADC? You said it yourself, his single target dps is increased when you combo. When you do not combo his R damage is absurdly low. They reduced his ability to do insane aoe damage with his ULTIMATE, not his W. They hit W ratio's and added mana back on Q to try and make people stop maxing it for waveclear. Again, you said it yourself, if nothing else, you could just throw out your ulti for free stacks...
Believe it or not, I've played more Vel Koz support games than in any other lane (op.gg EUW 0RedKing0). Ok so you don't max Q every single game, excuse my assumption.
"his W was max'd for wave clear, why would you take waveclear in lane with your ADC? "
To push the lane quickly? Maybe it's just me but as a support a skill I often need to employ to accelerate the game is fast pushing. Whether it's to get people under their tower to pressure them, or manage waves in general at various points in the game (depending on how the game is going). Wave clear is often a really helpful skill on a support.
Just because there's an ADC in the lane doesn't mean they can push as fast as is needed, or can push at all depending on their champion/mana availability etc.
"When you do not combo his R damage is absurdly low."
I can't really crunch the numbers now, the damage on just ult is definitely lower at early levels. However the new 100 (now 125%) AP scaling adds a lot of damage as you get a bit of AP in your build. This has definitely weakened support Vel Koz's ult on its own, but it hasn't affected my ability to succeed as support Vel -and he isn't designed to be a support, but if you're doing even half well you do tend to get some half decent AP anyway.
"They reduced his ability to do insane aoe damage with his ULTIMATE, not his W"
Full AP Vel Koz still has great AoE damage on his ultimate solo. Late game (i.e i've picked up some AP items now) support Vel Koz also. No it's not a bunch of free true damage, but that seems reasonable given how safe and easy it is to drop your ult on an enemy team.
You shouldn't be able to true damage shred multiple members of an enemy team with not much effort or risk. It was too low risk for too high reward.
"They made him more tedious to play, that is all."
This is subjective. Personally I find his new play-style more engaging and rewarding. Yes I have to focus more on landing my combos, but holy shit when you land a true damage ultimate your opponents health literally disappears in the space of 1 second. This is immensely satisfying especially when you're sitting on just a frost fang and sight stone.
"His reward for doing well before far outweighs the reward of doing well now. "
Not true. Old Vel Koz couldn't really do shit against tanks. A bunch of passive procs can't compare with full true damage on your ult. Whereas before a tank could let you kite him for a while before giving up, now he absolutely can't risk that you'll throw down your decent base damage, insane AP ratio true damage ult.
Feels real good to me that there is now nobody in the rift safe from being absolutely rekt by laser doom, regardless of how much MR they build.
"Good vel'koz players tried to amass combos and nuke as many players in a fight as possible, he was a sort of AOE ranged nuke, whereas xerath was the single target ranged nuke. Good Velkoz players understand how much more damage he did before, and how much less damage he really does now."
I won't respond to your "no true scotsman" logical fallacy. But hearing such arguments tends to be the death knell of intelligent discussion.
"If he actually offered up more reward for skillful play"
Being able to land your entire ult with true damage is a huge fucking reward. Can any other champions in the game boast an AoE, goes through enemies, 950 +125% AP (as of 6.10) true damage ult?
This has enabled Vel to actually blow up tanks in a 1vs1, something he was not so great at doing before even when his ult gave you passive procs. This new functionality is a reward for skillful play.
Not to mention his passive now scales with AP! Despite the small drop in per level base damage incrase, the new AP ratio adds a couple of hundred true damage to each passive proc that you never got before.
Again you're veer off into a "no true scotsman" logical fallacy.
We can agree to disagree.
If i'm being intellectually honest I cannot overall say that the Vel Koz re-work was a nerf.
To me he seems weaker outside of hitting combos (short trading and disengaging), but stronger and more flexible (in terms of being able to disintegrate tanks that he wasn't so good at before) on hitting a combo with ultimate.