2) Having optionable relievers will help the Mets out a ton
The Mets have four locks in their bullpen now following the Diaz injury. Those players are David Robertson, Adam Ottavino, Brooks Raley, and Drew Smith. The other four spots, at least to begin the season, will be a revolving door of a massive pool of players.
Stephen Nogosek is out of options, as is the recently acquired Dennis Santana. Other than those two, every reliever the Mets will have can be optioned other than Jimmy Yacabonis who is unlikely to open the season on the MLB club.
Having all of these optionable relievers allows the Mets to essentially always have a fresh arm or two in their bullpen. In a 162-game season, the value of this cannot be overstated.
Players only being allowed to get sent down five times per season definitely makes it harder for Billy Eppler to manipulate the roster daily like GM's used to, but the Mets should be able to do a good job fielding an at least partially-well rested bullpen most of the time.
As nice as it would've been for the Mets to sign someone like Zack Britton, that limits the flexibility a lot more for an older pitcher who is probably not much better than someone like Bryce Montes de Oca or Stephen Ridings.