It is extremely rare for any show to maintain a high standard of quality for more than a very few seasons.
I still like the show quite a bit, and I do respect the concept that the writers have allowed the characters to "grow up" into relationships, etc.
However, I don't think that the effort has been particularly successful. They have "shrunk" the guys' personalities to "make room for" the girls in the show, and that has left all the characters ever more shallow and less interesting.
Rajeesh, in particular, has been painfully downsized, and many of the best silly and obvious one-liners that would have been his now go to Bernadette instead. With him relegated to the sidelines rather than being the primary comic foil, the humor of the show has suffered greatly.
In fact, actual humor is what is most conspicuous in its absence now. The show simply isn't very funny any more. There are jokes and humorous situations now and again, but they are mostly just low-key and sad. I can hardly remember a plot with something like a road trip to a comic convention, or anything else strikingly nerdy.
