Every time a show changes, they lose viewers. DS9, VOY, and ENT all had their highest viewing rates at the beginning of their runs. ENT tried to shake things up a few times - at the beginning of seasons 3 and 4 - and paid for that with an early cancellation.
Stargate Atlantis started out much less popular then SG-1 - even if their viewing numbers were about the same, SG-1 nearly always won the "which show do you like better" polls - both here and at Gateworld. It's taken years for people (including me) to admit that Atlantis is actually pretty good.
Overall, people just don't like change. The public outcry when Ploto was de-planetized is another indictation of that.
Sadly, I'm going to have to agree with this. It makes it a bit rough on viewers like me who LOVE change in shows though. Voyager, SG-1, Enterprise, Malcolm in the Middle, & The Simpsons are but a fraction of shows I've lost total interest in when, after watching 3 or 4 seasons, it was still the same ol' nonesense over and over.