If multiple studios DIDNT own the rights, marvel would have simply been content making spider man and X-men films. Look at DC. One studio (Warner) owns the rights to all properties, and since 1990 you've gotten 7 Batman Films, 2 superman films, and a handful of horrific efforts like jonah hex and green lantern that were DOA. A decent wonder woman, flash, JLA, hawkman, doom patrol, shazam, whatever movie should have happened decades ago. but nope- batman is what sells so you get more of that.
There never would have been a NEED to go to the B and C Tier that was Iron Man, Captain America, Thor, and the Avengers- only milk the sure fire hits.
The only reason marvel still had the rights to those is because it couldn't sell them. all the other properties were considered to be more valuable, even ghost rider, punisher, and daredevil.
Different studios means marvel was forced to dig deep into their catalogue and make Cap, Thor, and I.M. relevant properties, and Avengers a household name. Given that the only drawback seems to be that some fans are slightly disappointed that they'll never see the "perfect" version of spider man they always wanted (and this would NEVER happen, Greed always wins out over vison) I think we're better off with marvel not having the rights to those properties.