Not really true. Take a look at the series averages:
Gavan: 14.9%
Sharivan: 13%
Shaider: 12.5%
Spielban: 11.8%
Juspion: 11.2%
Spielban and Juspion aren't that much worse than Shaider. Also, the "radical changes" didn't really do anything positive for the ratings, at least not right away:
Metalder: 8.2%
Jiraiya: 9.6%
Jiban: 9.1%
The ratings don't rebound back to their old, pre-Spielban levels until Winspector, which pulls at 12.8%. They fall back into Jiban's 9% territory around the time Blue SWAT is airing and remain there for the rest of the franchise.
You should also take into account materials advances. It's much easier to construct a suit that can remain shiny even during stunt sequences with modern materials than it was back in the early 80s. It's also easier to make them light up on command, thanks to LEDs.