2) Ryan Mountcastle
With Alonso out the door, the Mets were going to need another guy to play first base, and Mountcastle was a name that often showed up in rumors. He's never been a star, but as a decent hitter who has shown flashes of potential throughout his career, he appeared to be a solid, cost-effective replacement for Alonso, even if not the ideal candidate for the long-term role.
The issue with Mountcastle is that he's simply not good enough to replace Alonso. He's an obvious downgrade, but so much so to the point where it would damage the team's core. Replacing the heart of the Mets’ lineup with him would’ve been a shaky foundation for a contending team in need of stability -- not to mention he's no better a fielder than Alonso, who's never been great at first base.
Needless to say, this hasn't been a great year for Mountcastle. After 52 games in which he posted an OPS of just .628 and a wRC+ of 74, he sustained a right hamstring strain in a May 30th game against the White Sox that would have him missing significant time; he's expected to return from his injury in late July or early August.
If his hamstring problem had flared up with the Mets, you can imagine the disaster it would be in Queens without a starting first baseman for over a month. It's possible Mountcastle's injury wouldn't be a factor if he were on the Mets. Still, regardless, based on the production (or lack thereof) we've seen from him so far, it looks like they dodged a bullet -- especially with the way Alonso is performing.