Avimelech, I felt the same as you about upgrading to WP 5. I was worried that something was going to break, especially with s2Member, and there would be no way to fix it. And in my head, I imagined all of my existing members complaining, demanding refunds, etc.
But really, as long as you backup your database and your WP installation before upgrading, which I did, then if anything goes wrong, you can simply restore your database and replace the WP 5 installation with your previous one. Just make sure that you install the Classic Editor plugin and activate it before upgrading to WP 5.
I'm actually kind of glad that I went through this process because it forced me to learn how to backup my database, download it to my local machine, and get a local version of my WP site up and running on my computer. So now if anything goes wrong with my live site, I have my working local copy ready to go. And I can test out anything I want, including new versions of WP, without affecting my live site.