They've been amazing so far, so yeah, it's a valid concern that they may not be able to keep things at that high level. I just disagree with the mentality that something done now can adversely affect what came before it. Sure, the average overall quality of the films may dip a bit, but people do say things like "...and then they made <movie> which ruined the whole series!" No, they made a movie you didn't like. Ignore it, and enjoy the others. Simple as that.
The only exception I can think of is when a later movie retcons key elements in the series, and do it poorly or introduce something really stupid, to the point where you can't watch the older films without thinking about what is "really" going on supposedly. Makes it harder to ignore, for sure, but it can be done.