You should've seen Veigar pre-5.4, where his stun was not only instant, DFG's existence actually made his gameplay more assassin-like, and most of his damages are single-target. With these three in coexistence, it is not uncommon to play him as the "immobile assassin".
Because of how stupidly inefficient it takes for him to AP farm, many Veigar players omitted the "Q farming" mechanic and just use it as a way to bully enemy laners, even against assassins.
Veigar post-5.4 made him more magelike Q being able to hit two targets is actually pretty big especially in teamfights, and that dueling is not just "press buttons as quickly as possible" but rather "use some brain and trap THEN kill enemies". Veigar post-6.9 actually opens up more options for him to actually make the AP stacking actually worth farming for as well as encouraging him to trade with enemies in lane.
If you think new ult is a problem, his old one wasn't any better because:
- If you're an AP mid you might as well lose simply because of counterpicking. Think of it this way: Sona was reworked because she was the definition of "pick to win" where people she counters has nearly no retaliation, and people who counters her is pretty much an auto-win. Veigar was very close to that mainly because of his ult.
- With enough AP he can pretty much 100-0 squishies with a click of the R button. And again, huzzah for counterplay, even if you build SOME MR. That was the definition of assassination. Making the new ult an execute-oriented spell actually removes a good deal of that assassination playstyle.
In fact, a lot of mages have that "lock someone down, then finish him off with your combo" for the longest time. Think Lux, Annie, or even Vel'Koz. They don't have assassination playstyles, yet that's what they pretty much do. Veigar is pretty much one of those, but like those guys, he can teamfight just as well.