These past years Safari gets two releases per year, the big one with the release of MacOS in fall and a .1 spring release, adapting web technologies and sometimes extra releases tied to other Apple events and Apple proprietary APIs

I'm somewhat sceptical about evergreen browser, but that's mostly because I hate having the user interface changed under me without any action on my part. I would have hated to get the Safari 15 Beta just by opening my browser in the morning. A better update strategy seems to me to keep the big UI changes for a yearly update; but update the web engine somewhat more often, bi-monthly or so.