He has been the primarily (more accurately nearly sole) lyricist for more than 40 years. They are going to find a drummer and a lyricist that fits the expectations and brand of the band? One the fans will accept? And the long time, die hard fans are going to go for this? The long time, die hard fans (the ones who actually buy the bulk of the tickets) are going to stand for a Rush show without a drum solo.... or worse, someone other than Peart doing a drum solo at Rush concert? And please don't say "Portnoy".
When they went through their very extended hiatus after the tragedies, Neil made it very clear he was done and the fact he came back was something slightly short of miraculous. Alex and Ged did not work together on any musical projects as Rush during the hiatus, but released solo work. But this time they are going to change their mind? Particularly nearly 20 years later and older? Particularly given Alex' own health concerns?
Both Alex and Ged have made it very clear that Rush is a three piece band with three specific members. But they are apparently lying?
Here's the thing.... they may do a one-off festival show in the future (although doubtful). They may do a short residency thing (would be more likely given their perfectionist nature and a one-off has too many risks). They may even record together as Rush (although this creates expectations of touring, so more doubtful). But there is no reason to believe they aren't done touring.
I would contend that people who think Alex and Ged are willing to go on as "Rush" without Neil grossly underestimate the brotherhood among these men.