When 7 and 8 support was fully dropped (where the newest Steam client requires 10 or above to run at all), Steam clients on older OSes were not updated automatically. It's only if you manually try to update the launcher, it would "brick" your installations.
I agree that GOG's method will stand the test of time better (as we wouldn't have to archive specific versions of Steam), but for those with existing setups you aren't SOL right away when support ends.