That's a good point but odds are gmail won't be around in 20 years. The most popular email 20 years ago was aol later hotmail. Most popular search yahoo, altavista are gone. ICQ is mostly gone.
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
ICQ is very much alive, and popular in Russia for example.
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.
In that sense, Steam is not tied to a single hardware platform. They are a bit more platform agnostic and that means they can keep moving with the times. Steam doesn't require a custom steam PC using proprietary hardware.
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.
Nintendo is an interesting case, because while the stores are gone/going away (though there was a Wii->Wii U transfer period), a lot of the games themselves aren't gone, and what you already downloaded will continue to work so long as you don't delete it. More importantly Nintendo itself is still around (and will probably be in another 20 years -- there are many long-lived entertainment companies) it's just that they (and others who re-release on the new stores) want you to keep re-buying the same games over and over.
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.
MS are a special case as they keep the Xbox 360 store & library up for BC reasons, even adding older games to the catalog when they get publisher rights. I would actually trust them to keep it online. (And Sony recently changed their mind on shutting down the PS3 store and are keeping the PS3 download CDN up)
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.
