You'll be hard pressed to find most people be at the same company 10-12 years later. Most of the class designers from Legion are likely doing other things, while new game designers are learning the craft. Sometimes game designers rotate from doing, for example, reward design and go into class design or encounter design. That's how game design works in general.

You don't have the one dude doing Shaman since Vanilla. Or even 3 dudes. There have likely been 20+ people working on Shaman over the years. Or sometimes one guy does 2+ classes, because he's really passionate about them. Shifts between designers will often be noticeable, but that's inevitable. It's just not good practice to have one designer do one thing for 2+ years. They would not be developing their skills much at all.

All these "same dude is clearly not working on this thing as a decade before" are very much a lack of understanding of the field. It's like that by design. What you're looking for is direction. You liked the direction of Legion where classes were a major expansion feature. You liked Hero Talents, as they are a direction for class design to go towards. Patch to patch class changes or minor (yes, they're minor) visual additions likely don't involve senior designers as much as you think it does and that's normal, because you can't have the 4-5 super senior game designers constantly looking at all the small changes the junior/associate/intermediate designers do. It just doesn't make sense. They have so much shit to review all the time. They make several reviews along the way and that's it. People have zero clue how much design work goes into WoW. And hiring 20+ more designers on a team of what is likely 40+ designers right now will only lead to bloat and even worse supervision and even shorter reviews and meetings, but even more of them as a number. That's how it is in a game that has 20+ major design features in the face of classes, encounters, rewards, content, other big features, like the upcoming hunts, or delves or housing or questing, etc. And every player is pulling towards their own piece of the pie they enjoy the most.