Your rational only holds true if newbies are being turned away because upper tiers are on the ships. My bet is it takes a combination of newbies and repeaters to fill the ships week after week. And a lot of new berths are soon coming on line that need to be filled.
And when the next pandemic or recession comes along and no one wants to go near a germ infested cruise ship we know who the cruise lines will be going after to fill berths. Yeah, we upper tiers have long memories to fall back on.
If RCI truly didn't care about repeaters they would have scrapped the C&A program yesterday. And if they take away the upper tier bennies then there is no carrot to keep those newbies from trying different cruise lines to see if they like those "other" ships better.