I think one thing we tend to overlook is the emotional tie both Geddy and Alex have to the material with Neil. It wasn't like they were Geddy's songs, or Alex's, that the guys then recorded and played. Every song, with minute exceptions, was a living embodiment of equal contributions, and is made up of equal parts of all three of them. For 2/3 of the band to play without that final third I think would feel wrong to the two of them, forgetting entirely what we as fans might think. 41 years making music as a group with the same guys....it would be like sharing a time-honored family tradition with some new guy you happen to know. I don't see Geddy and Alex doing it. For them to play Rush without looking back and seeing Neil, I think would hurt them more than please them.
That's my take at least. If they collaborate on different music, that would be something else entirely.
I am also a purist, who feels Rush without all three boys simply isn't Rush, and never would be.