I agree that some of the Star Wars I-III effects and machines looked more modern and advanced than stuff in the original movie (and that lots of what they did was just showing off--the ridiculous, useless Podrace scene being the worst offense). The same was true of Enterprise, the prequel to the original Star Trek show: the equipment and technology looked more advanced even though it was supposed to be a hundred years or so earlier.
On the other hand, I still think a prequel would be absolutely a better idea than a real sequel. In the Dark Crystal world, it would make perfect sense for everything to look better and more advanced. What we see in the original movie is a time after one wonderful civilization has collapsed (the Urseks') and another civilization has been wiped out intentionally (the Gelflings'). I don't think it's a stretch to say that the environment (around the castle, anyway) has gone through years of devastation and overexploitation. So if we went back in time naturally everything would look better. Instead of the ruined Gelfling temple covered with vines, we would see a bright, clean temple in the middle of a bustling city.
I'm still holding out hope that it will be a prequel. There hasn't been much news about this movie, so I think there's still time for that shift to be made.