“In early April 2024, online play and other functionality that uses online communication will end for Nintendo 3DS and Wii U software,” Nintendo said on its Support website.
“This also includes online co-operative play, internet rankings, and data distribution.”
“if an event occurs that would make it difficult to continue online services for Nintendo 3DS and Wii U software, we may have to discontinue services earlier than planned.”
As for Pokémon Bank
Pokémon Bank will still retain its online functionality (presumably since it’s still used by players and uses online connectivity as its central mechanic).
Edit: Official Announcement with Q&A: https://en-americas-support.nintendo.com/app/answers/detail/a_id/63227/~/announcement-of-discontinuation-of-online-services-for-nintendo-3ds-and-wii-u
I assume that there is some fixed resource cost for the servers on top of the per-user cost. For example, they need to maintain a user database of every single single account that ever played online.
Also, systems that are designed to scale up well are often not designed to scale down (for example, the match making queue is sized for 10,000 users. It may be something that is not too hard to fix, but fixing it would involve rewriting code that they are not familiar with (all of the devs probably moved on to other work and likely many are no longer with the company) and is going to introduce all sorts of new scale issues (for example, a server needs 1 GB of free memory to run a vulnerability scan. If you cut its memory allocation by 50%, that might not be the case and you will not know it until the 3rd Tuesday of the month when the scan is scheduled).
It is possible that it costs Nintendo almost the same to run online match making for 10 players as it does for 10,000.