I think it's a tough line to walk, as evidenced by this thread.
When they don't update something for a long time (as with these road signs) it stops being 90s and starts just being Disney for a lot of people. I've seen people who only went to Disney for the first time in the past few years upset about the change because they associated that specific look with Disney instead of a design era.
Disney parks do trade on nostalgia pretty heavily, so there's a risk involved in doing anything that upsets that nostalgia. That's not to suggest they should just everything exactly the same forever (and we're talking about road signs here, not something fundamental to WDW), but compete overhauls rather than iterative changes are probably a mistake in many instances.