I understand the reason they wanted to charge commercial applications for use of the API was that their costs had gone up, because they had moved to AWS to improve their reliability and performance. I don't know if you noticed, because MC didn't highlight it, but it wasn't unusual for TVDB to fail to respond to requests on the API. They claimed it was due to load. Of course for casual users who actually use the web page they can generate income from advertising. But for applications, commercial or otherwise, that use the API they don't get that chance. Users of applications never see the web site.
But I think you are perfectly correct in that commercial applications aren't going to offset their costs enough to make a difference. Particularly when they just shift, as JRiver have done.
I'm appalled that they now want to charge me to use it.
Tell them. Share your other thoughts. What have you got to lose? In fact, if the situation continues, you effectively lose access to all the work you put into the database already.
Charging users who do not actively contribute would seem like a better idea, as long as they allow people who want to contribute to do it. They hold tight reins on their data, as far as I can tell. I've never contributed and would be willing to pay around $10 a year for access. I don't know if that level of funding would cover costs, but I suspect it might. It would just need the applications, all applications whether commercial or not, to add in a user login process for using the API. But that must be there at some level for developer API now.
So have at them! TMDB is okay, and probably will get better, but it misses pretty often as well.