A good question, posed by The Athletic:

The disaster scenario for the Phillies is if the Mets, Nationals or Braves sign Realmuto.

The Mets, under new ownership, could offer a deal the Phillies might not be willing to match if they’re looking to significantly reduce payroll for the next season or two. Andrew Knapp proved he’s a good backup catcher.

But going from Realmuto to a combination of Knapp and Rafael Marchan/veteran catcher is a significant drop-off. The likelihood of the designated hitter sticking in the NL helps mitigate concerns about the effectiveness of Realmuto, who turns 30 in March, in the latter years of a four- or five-year contract. It helps he can play first base, too.

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When the Phillies gave up a prime asset in Sixto Sánchez and a still-developing catcher in Jorge Alfaro for Realmuto as part of the February 2019 trade, they kicked open their window for postseason contention and a World Series title. It also signaled Realmuto would become part of their winning core. Failing to retain him would be a big blow to the Phillies’ short- and long-term goals.

https://theathletic.com/2106638/2020/10/02/ten-things-we-learned-about-the-phillies-this-season-and-what-it-means-for-2021/