That's the thing about Switch Online that will not age well. With the Virtual Console you could at least download the games and you had a permanent digital version of it on your SD card or saved on the console. With Switch Online you are just renting the available games, you don't own the titles digitally and can only access them if you have an active online membership. But once they stop supporting the Switch in the future there won't be a way to access those games on the console. The only thing I can imagine is that they continue using the same servers with future consoles, that way the Switch can still connect to it.