Ultimately it's up to managers to start doing it, and who is willing to go there? Pretty much all managers are still old-school guys at heart, even somebody like Maddon. You're starting to see some bending of the traditional RP roles, especially in the postseason with guys like Andrew Miller. I think we'll get there eventually because I think you're right. Starters are going shorter and shorter, and we're starting to see some managers willing to go beyond the (actually modern) rule of no more than an inning at a time. Especially with the rise of guys who just get 1-2 outs as well, something has to give at some point so that your longer relievers aren't worn thin. The answer is to have more guys who are able to go more than an inning at a time.
I'm not sure we're anywhere close to a time where a team will go with the '3 pitchers pitching 3 innings' idea, but I do think we'll start seeing some relievers who typically go 2+ innings every few days.