Not worried about the players. This isn't basketball, soccer, football or hockey. They stand around most of the time anyway. They're fine playing 15 innings and then playing a day game (if need be) the next day.
Yes, in some situations it could be more taxing in the regular season. But again, you have a AAA team to call guys up from if need be in the regular season. You don't have that luxury in the playoffs. On average, it's going to be more taxing in the playoffs.
You're also solving a problem that doesn't really exist. Games longer than 12 innings:
The 2019 Cubs played games that went 13, 15 and 15.
The 2018 Cubs played games that went 13, 14, 14 and 17.
The 2017 Cubs played games that went 13 and 18.
The 2016 Cubs played games that went 13, 13, 13, 13 and 15.
The 2015 Cubs played games that went 13 and 13
The 2014 Cubs played games that went 13, 13, 13, 16 and 16.
The 2013 Cubs played games that went 13, 13, 14 and 15.
The 2012 Cubs played one 13 inning game.
So the Cubs played 11 games that went into the 14th inning in 8 years.
It's really not that big of a deal and you're making it sound like teams play 15+ inning games every couple weeks. Call up a AAA SP and a RP and you're good to go. It's fine.