I'm sure you get this Wok, but for me, this idea is based on the thought that starting pitchers are throwing fewer innings these days because the #'s support that strategy (ie, don't let the starter face the lineup a 3rd time + drastic improvement in the quality of the middle reliever) vs. the idea that pitchers are throwing fewer innings because their bodies can't handle it. If that's true - then let them continue throwing less innings/start (which they are already doing) - but start them more frequently.
I think the premise with this strategy is you wouldn't necessarily be asking starters to throw even fewer innings/start (and thus adding innings to the bullpen). It's simply instead of having 5 guys average 5 innings per start (approx. 160 innings over 32 starts), have 4 guys average 5 innings per start (approx. 200 innings over 32 starts). That way the bullpen throws the same # of innings, but has an extra guy in it to help.
200 innings used to be a standard or goal for starting pitchers.
In 2010, 45 starters reached 200 innings. In 2017, only 15 did.
In 2010, 63 starters reached 190 innings. In 2017, only 19 did.
In 2010, 73 pitchers reached 180 innings. In 2017, only 35 did.
If this trend is all about strategy - and not about health, then this is a legit thought. Unfortunately, I fully suspect it's a bit of column A and a bit of column B - which clouds the situation. The other obvious issue is when you have an Ace who can go through the lineup a third time.....you can't have him throw 7 or 8 innings and then pitch again on 4 days rest. You'd probably need some swing man, or AAA starter that could spot start when needed...
I don't think it's a terrible thought - interesting to discuss, yet unlikely we ever see it.