Yeah but once he retires and becomes a coach he takes a massive pay cut. If he has a shot at more service time anywhere, he has to take it, plus he probably wants to play. He's not going to refuse to report to another team on their major league roster. And he wouldn't be the #3 catcher. Or, if he is, he'll still be a big-leaguer. If he goes on waivers again, the Phillies could get him back. Now, if it turns into a Buddy Kennedy situation he may just retire rather than be Erik Kratz.
And who knows, maybe he'll make the Phillies. The other bench options (Moore, De La Cruz) aren't on the 40, and they could also figure on having an easier time slipping him through waivers a few weeks or a month into the season. It certainly would give Thomson a nice option for PH with Marchan or Realmuto too, if they did it. My guess is he's still playing in Allentown on their Opening Day.