Myspace is gone. Facebook is dying slowly hopefully to be replaced with some form in the meta world.
To make it 18 years is special but it's going to take a lot of reinvention to keep up for the next 20 years. They have the best odds but so did myspace
Hotmail launched in 1996 and GMail didn't get popular until at least 10 years later. (IMAP was added in 2007~ and the beta was lifted in 2009 iirc).
That's a lot of time to be relatively dominant, but they're reminiscent of zombies these days.
Hotmail transitioned to Microsoft outlook.com, and you can still use your hotmail domain email, even if you can't register new ones.
As far as I know aol mail is still available.
Myspace is still live, including all the old pages and you can probably listen to music uploaded to it in the past as well.
Losing popularity does not mean shutting down.
Yahoo search and Altavista are not the same, since you did not upload information to them nor bought something. Though Yahoo search is still alive anyway.
A good example of these online store going down would be the Wii and Wii U/3DS stores. The Wii store is already gone - it lasted 13 years.
The Wii U/3DS stores will no longer accept any payments starting 18th January 2022 as a first step towards shutting the services down. Wii U is 9 years old, 3DS is 10 years old.
Only yesterday a friend of mine fired up his Xbox 360. While the online system is still running it is a question of how long until MS will simply drop the entire thing even if it is a very minor thing to run.
As for Nintendo, I'm unsure but optimistic -- the Switch is their first console that seems planned to be (able to be) used with 100% digital purchases, I'm not sure if games will be buyable forever but I bet they'll be downloadable.
I have a similar desire to paxys in that I want to be able to play most of my games in 20 years, where such a concept is even valid (I do sometimes enjoy multiplayer games and other things "of the moment" where I have no expectations of playing the same game tomorrow due to a sweeping patch or upgrade or shutting down, let alone in 20 years). It's just that digital store risk, especially of Steam, is the least of my worries.