i think the single player model is fading for a large number of users.... any game should have the option
for coop, even if the cutscene/story has to be toned down for the coop experience, you can still have the full experience in single player and you dont have to whine about the addition of coop (not a jab at you, just in general)... imagine how much better fable 2 would have been if the coop was given to us as promised (not some broken, screen sharing henchman gimmick)... or take mass effect for example, you're always toting 3 players around with you... would you really rather rely on the AI vs your friends? mass effect would have been a much better game (which was already amazing IMO) with coop.
a very large portion of players and customers want coop, if it doesnt affect the single player experience, why not broaden your customer base? honestly i will buy ESV either way, but these days i'm specifically checking reviews on co-optimus because frankly i typically have a better time in coop.
borderlands and fallout are very similar and completely different in their own right, but i'm having much more fun with borderlands.... and i WAS the guy with hundreds of hours in ES3, ES4, ETC
ES4 already had scaled difficulty when you swapped difficulty levels on the fly.... it wouldnt be hard to implement a scaled difficulty for multiple players (like borderlands does).
and while i did say it would be ok for ES5 to be somewhat instanced (just large, bustling, busy cities where you could show off or trade weapons and gear) it doesn't HAVE to be that way.... but i was just saying, if they have parties of 4, these little towns are basically your lobby for matchmaking (and trading of course)... when you leave the town/city, matchmaking is complete, and you're in a normal coop game with whomever is in your party.
and to explain a little more for FJ as far as how much of a shooter/rpg borderlands is...
Halo - Bioshock - Borderlands - Mass Effect - Fallout 3 - Oblivion
of course halo has no rpg elements, and oblivion has no real shooter elements (unless you want to count archery/spellcasting