I think the real argument here is that baseball, especially hitting, has changed. Today, there isn't a guy in MLB who can't drill a 94mph fastball with regularity. Pitchers keep ramping up velocity but it has side effects: you can't go as deep into a game if you need to ratchet your arm speed to 96mph for hitters 1-9. Doubly so because not only are hitters better but they're far more selective with their swings nowadays, running up pitch counts early. Therefore, we've seen a decline in starting pitching IP. There simply aren't that many guys who can effectively get through a lineup three times because, unlike previous decades, they no longer get to phone in the bottom of every lineup (or, in some cases, half or more of every lineup). So teams are figuring out new ways to get those outs. The opener idea has its merits for that reason.