I agree with the OP.
Even in 1995 or so, I would look at old episodes of TNG and be startled by the difference.
Same thing with Star Wars compared to The Empire Strikes Back.
While Empire and Return of the Jedi look similar. Or of similar production quality I mean.
I completely agree about the stark contrast between SW and ESB. The light sabers in the former looked like fluorescent tubes. In ESB, however, they look as good (IMO, better than) those in the prequels. And RotJ looked about the same. And that was only a three year difference.
I still maintain, however, that the first few seasons of VOY's space FX looked bad. I never knew why until yesterday I saw who was handling the FX. It was a different company for DS9, and VOY switched around season three or four, which is when it started to look great.
I will agree, though, that the sets and overall production quality were the best of the TNG-era shows.
There was only a seven year gap between TNG season one and VOY season one. I think those seven years made a much bigger difference than seven years today would; ie, a 2003 vs a 2010 TV show.
But don't you agree that the difference between later TNG and its first season was even greater than the difference between TNG and VOY overall? I thought by seasons four--seven, aside from the dated sets, still look pretty good. TNG season one looks like there was a 15 year gap betwen it and say, season five.