It's a sad day but it's been long coming. That said, there are still ways to connect to the GTS and get Mystery Gifts and do ACWW/ACCF multiplayer modes, so I cannot imagine that there'll be ways to still do that for the 3DS.
The thing that I'm not sure how people are going to deal with Pokémon. Some people like to collect all ribbons. For some, that means getting a Pokémon in Gen 3, getting all possible ribbons, transferring them up to Gen 4, repeat, Gen 5, repeat, Gen 6, repeat, Gen 7, repeat, Gen 8, repeat, and finally Gen 9.
It means that this fun quest people do is going to be a lot harder, if not impossible. There's a Gen V softwall, if you don't get PokéTransporter before Bank goes down, and a Gen VI hardwall, where if Bank shuts down, you cannot move Pokémon further into Gen VII. As someone who enjoys doing this myself, it's sad knowing that one of the things I enjoy doing with Pokémon is gonna be over and that I have a timelimit to wrap up what I want to do.