2) Alvarez's defense has been better than advertised
One of the reasons proffered for keeping Alvarez in the minors was that his defense wasn't major league ready. Luckily, the reports of Alvarez's defensive deficiencies were much exaggerated.
In 150 innings behind the plate, Alvarez has committed only one error. He's struggled to throw runners out, but with the new rules, that's been a league-wide issue. Baseball Reference says that Alvarez is on pace to save 24 defensive runs above average, which puts him at eighth in the league.
The Mets' pitching has hardly been impressive, but that can't be placed at Alvarez's feet. The Mets have struggled to get guys out all year, no matter the catcher. In fact, Alvarez has done a superb job helping his pitching staff, ranking eighth in the league in pitch framing according to Statcast.
There's no way to know if Alvarez can continue being a top ten defensive catcher, but it's clear that he's not the liability in the field that he was painted to be. With his burgeoning ability as a hitter, if he can just be average defensively, that would be a big win for the Mets.
Being able to stay behind the plate also means that Alvarez doesn't need to DH, a fear some Mets fans had. This creates an easier path to the big leagues for Mark Vientos, a big bat who doesn't have a natural position to play in the majors.