We’ve seen Baez improve massively as a hitter once he started playing every day. We’ve seen him improve massively at short once he stayed there. Is there any reason not to think that would continue? The fact is you’d be wasting one of Baez’ best traits (his arm) at 2B. And you’d be hiding Russell’s defensive weakness (also his arm).
Also - Baez is an automatic start every day and Russell is not (or at least he shouldn’t be). If you put him back at SS, that means Baez has to move every day based on whether Russell plays or not. Whereas if you leave Javy where he is, no extra moves required - Russell either plays 2B or sits.
Seriously, it’s astonishing that this is still even a matter of uncertainty.
It is also true that, as would be expected, the number of games Baez has played has increased as his performance has improved. We can argue the "chicken and egg" thing, but there is hardly enough data to determine which is responsible for which.
Baez performance improved substantially last year over the previous year, and he played the majority of games at second base. The fact remains that there is no reason to think that his offense is in the slightest influenced by the position he plays, or the regularity of that position.
I don't think that Baez has improved substantially as a shortstop merely because he is playing the position exclusively. Yesterday, he made a great play, crossing to the shortstop side of the infield to throw out the runner, even though, because of the shift, he was essentially playing second base at the time.
Moving every day has not seemed to affect Baez in the slightest up until now, and there is no reason whatsoever to think that it will do so in the future. If it turns out that his performance DOES suffer after a change, there is no reason why the change can not be reversed.
I find it astonishing that you come to such certainty of your position with so little data to base it upon.