You haven’t answered my question. Why is Kelly better than McGuire? Is he much better defensively? Kelly is a lifetime .229 hitter. McGuire is slightly better at .251. What Kelly has done early in the season is way out of his norm.
The fact Kelly was on the Cubs opening day roster tells you something about what the Cubs think of the 2 players. Plus you are deluding yourself if you believe the Cubs will send a player down to Iowa whose making $5M a year for 2025 & 2026. Whereas the Cubs are paying McGuire $523k.
Sure Kelly has only been a .200 hitter in May/June & if he's a .240 hitter, my guess is the Cubs would be content. But if not, the answer is Amaya getting a higher share of starts once he returns vs. keeping McGuire over Kelly.
Personally, I'd keep McGuire and DFA Bruhan when Amaya is ready to return. If the Cubs don't want to keep 3 catchers, then maybe there is a trade market for McGuire to pick up a utility infielder.