Fans need to realize this as a fact. I think nearly every team has a few linemen that aren't really up to facing the d-lines in the league nowadays. And most teams don't have two solid tackles.
Speculation only, but I wonder if more kids prefer to go to the defensive line, where the glory is, versus O-line trudgery. Certainly even in college you see more attention given to DLine speed and sacks then good protectors.
That's why scheme is even more important at this point. The playcaller has to design ways, be it play fakes, backs chipping, or max protect, that increase the offense's ability to hold up the pass rushers.
On the point about the drought, I doubt it's getting better any time soon.