When an original author (or a company) finishes a huge work, it gets the attention and creates a fandom. If a style is very simple, it's becomes simple to create the in-style fan-arts. The in-style fan-arts are very immersive in terms of an original universe, so these works are also popular. But does a fan-artist really want to lock into a very simplistic style of the work?

Sure, if he really likes it this much. But the original author moves on to the next work and (probably) experiences the growth, while a fan-artist turns back an sees the same thing repeated over and over. I saw the artists who reached the style they aimed for and still really wanted to keep growing, and I can't imagine the blunt force of realization of self-inflicted stagnation of the artist who was previously just happy to keep drawing in the original simple style.

So, the simple original works deserve the immersive in-style fan-arts, the artists deserve growth, and these relationships should fleeting like a holiday romance. Or at least i think so. Just... when you can't improve on your ability of mimicking the original anymore, try to improve over it. Make it look like how it will be remembered, not how it actually looked.