Even though that's true / rational, most people and consumers don't behave this way, especially when it comes to consumer tech.
If Apple announced a 15" MBA with an M3 chip around the same time / shortly after a Mac Pro w/ a M2 "Ultra+", irrational or not, it would probably make those Mac Pro users feel sore about it. Doesn't mean their Mac Pros are obsolete, old, or suddenly no longer efficient. But if you're dropping $10k+ (?) on a Mac Pro, you don't want to feel at all raw/sore soon thereafter.
So I think very unlikely Apple would do that, especially to their highest-end consumers.