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.