TNG, DS9, and VOY all had several 26 episode seasons, but I can't find many other shows from the time period that did. 26 seems like a lot.
I believe The X-Files was another show that aired tons of episodes. When that show was at it's peak in the early seasons, they were airing 26+ episodes a season, I bet. I'm too lazy to go downstairs to check my DVD sets, but I remember some of those seasons being really long. But the quality was there.
As for the main topic, I'm sorta with Temis on this one. If a show is good, it's good, and whatever it will be replaced with will likely not be. So I'd rather have more episodes, even if they are 'filler', since 'filler' of a good show is still better than the best episodes of most crap on TV.
And I'm not sure that mediocre shows would benefit by a shorter season. If a show has a weak premise or is being poorly written, poorly acted, or whatever, it will likely be so whether there are 20 episodes or 13. I mean, can poorly conceived or acted characters actually improve with fewer episodes? I don't think so - I just think there will be fewer episodes for people like me to NOT watch.

The key thing to remember here is that the networks still have 52 weeks a year to fill, and a finite bucket of advertising dollars to do it with. If shows are shorter, they will also receive smaller budgets, it would seem to me...because that same bucket of ad revenue STILL has to cover 52 weeks of programming. It will just be with more shows.
In fact (and this is the accountant in my speaking), I would guess that if shows got cut down to 13 weeks, budgets would get slashed by A LOT...because all the shows they will now have to generate to fill the time have their own 'overhead' (back office costs) associated with them that cannot be avoided (that's why they call it 'overhead'). And everyone knows that economies of scale are only there...if you have SCALE. With shorter shows, you won't have any scale at all, and the administrative costs of just 'show running' would be a lot higher for the network overall . Thus, less money for what we actually see on the screen: good actors, good writers who write good scripts, sets, special effects, etc.
Or did you guys think that these shows would get the same budget for a 13 week show as for a 22 week show? Because I really doubt that can possibly be the case. I mean, the math doesn't work.