I'm not so sure it would even pay for itself to continue running at some point. It's not like you can just pay to keep the servers running. It's connected to the internet and surely running on a software stack with dependencies that will have CVEs, or language version lose LTS, libraries dropped for support, etc. and need upkeep. How much money in sales could the Wii Shop channel have been pulling in by the time they shut it down? I can't imagine much.
Firstly, I think you're vastly overestimating these costs. Server resources are pretty cheap. They could probably overlap with online infrastructure of more recent hardware. They probably could also budget a small amount of the dev time to fixing a few bugs with extremely low priority, only the most high severity items prioritized beneath servicing modern stuff.
Second, AFAIK they are running those servers right now with no revenue. They blocked new purchases, but you can still re-download existing purchases. I googled around and it seems like that's still the case. I do not know how long they will maintain that.
You also have the customer support overhead. People will demand refunds. They will claim the purchase doesn’t work on their broken hardware. There will be fraud where people use the service to test stolen credit card.
There is also the staffing element. Maybe the engineers or product people are interested in resourcing a product with no career growth.
There is a hefty on-going maintenance cost for these stores.
There was that recent Youtuber that spent a few tens of thousands of dollars on every eShop title before they shut it down, so they made at least that much, but on the other hand, it’s all archived now…
