I agree to a certain extent. If they make it a practice to re-open the Ex waitlist, then they fully undermine the plan you outlined. If it happens frequently then folks will wise up and not jump on the Regs aggressively, and just wait for the Ex WL to re-open. My intuition is they likely data mined their WL order history and improved their model for shutting them down sooner.
Reopening the WL doesn't totally undermine their strategy. Let's say they reopen Thanos and Bane. Those will fill up very quickly and not everyone who already ordered a REG will be able to jump on the WL. Those that miss out in that small window to hop on the WL will still have to hold on to the REG. SSC will move more REGs that way regardless. Also, let's not forget there will always be collectors that don't want to take a chance of missing out and lock down a REG no matter what tactics SSC uses.