The graphics are simple on purpose - because the game is essentially infinitely scalable (theoretical max map size is 3x surface of the Earth), simple graphics are super important for not lagging servers and clients to hell.
He's said in interviews that the improvements he wants to make are improvements that make the game more fun. He's not going to add features that won't somehow add fun, and if he finds that something isn't fun, it's out.
Because the graphics are purposely so simplistic, people are okay with that. As soon as you upgrade them and start to make them more modern, you start getting compared to other modern games like WoW, etc.
Keep it simple, people won't worry about the graphics.
I also think he avoids any issues with the 'uncanny valley' of near-realism in games. If something isn't going to look exactly like a pig wandering the mountain slopes, why not make it look like a Lego pig and be done with it?
The graphics are fine. I'd rather see improvement made in speed of rendering (even on a decent machine, setting "fancy graphics" and a "far" viewplane is a recipe for low framerates and lag) before trying to change the look.
He's said in interviews that the improvements he wants to make are improvements that make the game more fun. He's not going to add features that won't somehow add fun, and if he finds that something isn't fun, it's out.
Because the graphics are purposely so simplistic, people are okay with that. As soon as you upgrade them and start to make them more modern, you start getting compared to other modern games like WoW, etc.
Keep it simple, people won't worry about the graphics.