We are well into the plateau phase. Smartphones has we know them started back around 2007 and we kind of got used to huge and dramatic increases in features, functionality and performance each and every year... sometimes even shorter. Every new generation, people picked up the new units and though "WOW!" But that rapid improvement wasn't bound to hold up. I think the turning point was around the Galaxy S3 generation. Once that gen of phones got dropped, the perceived pace of advancement slowed fairly substaintally.... CPUs can only get so much faster, screens can only get so much better, etc.
So while some might be disappointed with the current crop of phones, I look at the lineup and I'm impressed. I mean, we are up to our eyeballs with great devices.... even the 'cheap' phones today are infinitely more capable than the top of the line units we had a few years ago.