Now I'm curious to ask AMEX whether I can apply the Aeroplan Plat (and get 50K Aeroplan) but waive the $499 fee since I'm willing to renewal Regular Plat at $699/year

I think it'll be a no-go though, and I don't want to transfer all my AMEX MR into Aeroplan if I'm cancelling the $699 Plat

or is the plan going forward for everyone to "churn between Plat and Gold every other year" just to keep 1 active AMEX MR account open