Right, I was replying more to Laura's musings than to your OP and should have realized sooner that your focus was on the OP.

RE that point, the latest video by JJ Couey litigates whether Delta's ascendancy over other variants is due to previous infection immunity escape pressure (Trevor Bedford's proposal) or vaccine immunity escape pressure (his view). I think it's the synthesis of both. https://www.twitch.tv/videos/1159285424?t=00h11m44s