Realmuto is a better player than Springer (or it's extremely close either way) though and he's 1.5 years younger. So there's no real reason to think that Realmuto won't last but Springer will. There was also just an article the other day about how the catcher breaking down thing is largely a myth and dependent more on player type than position. Not to mention, regarding position, Springer is 31 years old and I believe there was one or zero CF this year at age 33+ so he's just as likely, if not more, to break down and/or have to change positions.
Rather than having McCann at C and Springer at CF I'd rather have Realmuto at C and Bradley or Pillar in CF. That's still one expensive top guy and one cheaper guy at the other position (not that they will be super cheap but not the same level as the others).
Take these numbers for what it's worth but here are projected median crowdsourced contracts for the listed players (
2021 Top 50 Free Agents)
Realmuto 5 x 22
Bradley 2 x 10
Pillar (not listed)
Springer 5 x 22
McCann 2 x 6
Bauer 3 x 29
A negligible difference between Expensive C/Cheap CF and Cheap C/Expensive CF