He HAS shown some growth in his game this year. He's a lot more responsible defensively, and he's actually developed some chemistry with his linemates (Barbashev and Paajarvi or Sanford). I think a lot of that is Barbashev playing well. Maybe having a fellow-Russian on his line is buoying his spirits.
If I could decree his path, he would sign with the Blues and play big minutes in the AHL next year. Maybe a full year like that. I think if he could emulate Paajarvi's path it would be the best option for him to become a contributing NHL player, at least a solid 3rd line contributor who scares opponents with his speed.
But the problem is the 2.5M qualifying offer. He's not worth that, and I don't think the Blues will pay it. They may sign him for 1.5M or 1M. But if he is available as UFA is there someone else who'd take him at a better price? At what point will he prefer the KHL? Seeing Paajarvi salvage his career (he looks like a solid 3rd line player now and has transformed his game in the AHL) may help Yakupov to be more patient in this process.
I hope the Blues find a way to keep him, but he's probably not worth the cap hit and investment of a roster spot since he's not effective in a 4th line role. You want your spare forwards to be more versatile, and he won't grow if he doesn't get enough minutes. Tough situation, but that's where he is now.