The downside to this would be that, if Realmuto's decline continues (which is likely), this kind of extension will tend to "kick the problem down the road," rather than addressing it. If they extend Realmuto, they won't look for a replacement, as opposed to a backup.
If they get good offensive production from the rest of the lineup, they could carry a declining Realmuto (albeit not in the middle of the batting order). But if they have a light-hitting CF, and a mediocre-hitting 2B, and a RF who's flirting with 0.0 WAR...it starts to look like a club with half a major-league batting order.
If they don't see improvement from Rojas and Stott, they're going to have to decide who to "move on" from. There's been a great focus on Bohm, but the regulars (those who are "qualified" per MLB) with the worst OPS are Stott and Realmuto - and Realmuto is the one with the big contract, who will be a FA this winter.
Would you QO Realmuto, if you were the GM? Not that they can, of course; Realmuto was QO'ed in 2021, so that's not going to happen this winter. But would he be worth a QO contract?
I would want a relatively cheap one-year deal (what is 1.2 to 1.5 WAR worth these days?), with a club option (and minimal buyout) for a second year. I suspect that, in the free agent market, somebody would outbid that, given that free agents tend to get paid for past performance.
