There seems to be a feeling in some quarters that modern sf is not what it was; indeed, I suppose I feel it myself to some extent, although there are still good books being written in the field.
I reckon the main problem is a general loss of optimism about the future. Back in the 1940s and 1950s, the future was seen as a fairly simple place, in which humanity was destined to go out and dominate the galaxy as it had already dominated the Earth. Eric Frank Russell wrote stories in which lone human scouts baffled and outwitted hordes of aliens unfortunate enough to lack human intelligence. These stories were a bit childish, but fun to read; people enjoyed reading them and came back for more.
These days, most people seem to suspect that the future will be complex and threatening, and sf authors struggle diligently to give a convincing sense of just how complex and threatening it will be. This is a worthy effort, but I wonder how many casual readers want to spend their spare time reading about a future that's even more complex and threatening than the present. Some readers doubtless appreciate it, but will sf retain a mass audience this way?
I'm not sure what the solution is to this problem, but in the meantime I see authors and readers (myself included) increasingly turning to alternate-history stories, in which we can read about societies other than our own without needing to venture into the unappealing future.
I hope this is merely a cyclical phenomenon, and that people will become cheerful about the future again in due course.