That's a question everyone has to answer for themselves. Do you want to continue to move up the levels of whatever sport it is you do and be competitive? One can continue to improve their riding and general horsemanship on just about anything so long as it's sound, but if you want to actually follow a progression from one level to the next, you need a horse who is physically capable of what is being demanded of it at each level.
I see it as a bit of a false dichotomy between a horse you can "improve" on and one you can't. Most of us can stand to improve our position, the timing and finesse of our aids, the softness of our horses and so on. You don't need a fancy horse for that. I myself have made a huge difference in how my horse goes by sorting out some issues with my riding and hence have found a well of impulsion and power I can now play with which wasn't there before. Don't need a new horse. Just need to be less crap at riding the one I have.
However, if improvement to you also means jumping 5ft or doing FEI dressage, then you may need the fancy horse.
The other way of looking at it is that if you are indeed still working out issues in your riding ( and I don't know if anyone on this thread is or isn't, but if you are), then work on those with the horse you have. If and when you get a fancier horse, you'll then screw up its training slightly less. The flashiest warmblood in the world isn't going to compensate for a unbalanced, uncertain rider who is still learning how to maintain a soft feel.