That's it pretty much.
I wonder if Steam will get hacked if they shut out XP users. The motivation would be there. All we need is our backups on a hard-drive and offline activation.
I never used Steam under Win9x so I wasn't even aware that a Win9x client once existed until I started this thread. I think killing support for Win9x was probably easy for them. Most of the games from that era were originally released on CD or DVD media. Steam came onto the gaming scene rather late for Win9x users. Also, by the time Steam went online, there probably weren't too many Win9x gamers left. The outcry from shutting down Win9x serivce was probably minimal.
XP, on the other hand, was a very popular OS for 13+ years. Digital Distribution became a thing during the halfway point of its lifespan. It still has lots of users and compatible games even today. I think there would be quite a howl of discontent should Steam drop support for XP without making a legacy client available.
I'm already wary of buying games on Steam, but that hasn't stopped me from building a library of over 200 titles. Dropping support for XP without providing a workaround would ensure that I never bought anything from them ever again. They've got to be aware that more people like me exist and that it would be very bad for business. It can't be too hard for them to provide a legacy client and say "Here's a legacy client for XP, but we won't support it. Good luck!".