It's no different to most seasons.. If United where in Liverpool's position now, we know for sure this thread would not exist.
In the years United dominated, Liverpool where often floundering behind in the league, yet the league wasn't considered weak then. For a lot of those years too Chelsea where not the factor they are these days (and yes they are still a good team), same with Manchester City. But again - no one would have been claiming it a weak league then.
For years it was a 'big 4', how it's the 'big 6', so that suggests the league is stronger in recent years, or else why the need to change it to accomodate more teams.
The premier league like most leagues has it's negatives, but it's still a very strong league, even the smaller teams can often compete at a high level and have quality players. Plus teams like Leicester and Wolves are now very strong and pushing some of the bigger names as well, so that's surely a positive, and not a sign of a weak league.
The league is still very strong, just that 2 teams are exceptional - and that is something to aspire to, rather than ignore, although maybe not City's way of doing it, as that would require unlimited funds, which most teams don't have the pleasure of.