I think one of the biggest reason for the pushback against this proposal was, ultimately, because it would have made a bad system worse.

We need to move to “on first use” thumbnails, so that WordPress generates only the thumbnail files that are actually needed.

Some very smart folks have been proposing this for years. There were also calls for it in the comments of the WebP “Proposal” back in March, and throughout the past six months. I hope Matt and the lead team will give this some serious consideration and encourage some effort on this. Of course it would be a huge undertaking, but if we can improve this system, it would be a huge win across the board (saving significant processor usage and disk space, and making backups/restores easier) — and it would make plenty of room for conversion to other formats, whether that’s WebP, AVIF, or whatever else comes along.