That's the problem with being a good team.
If you have glaring holes, it can make sense to overpay to fix one, going from 0 WAR to 3 WAR.
But if you're trying to upgrade Rojas, doesn't make sense to pay $20+M to go from 1-2 WAR to 3 WAR.
If Painter wasn't healthy, it might make sense to invest real money in another SP, going from -1 (Walker) to +2.
But if you think Painter gets 20 starts, and maybe another kid emerges, now you're spending to upgrade 10-20 starts overall. The value of that upgrade is diminished.