Yep, Framework (as far as I remember) intends to even create a public marketplace for people to sell custom-made addon cards. They have the dimensions on their github and make it pretty easy.
I personally doubt chess will fade. As far as I can tell, the enjoyment of high-level chess is less to do with reaching objective perfection, rather it's more the "thrill" of seeing a _human_ who has poured their life into the pursuit of improving their game. It doesn't really matter that a computer can beat them, we care about them because people like seeing other people's talents.
And on a casual level, nothing changes if the best in the world is a human or computer, so the casual player isn't particularly affected.
While the client-side has input validation, it appears that the server-side does not, as I can edit the request body freely and it'll return accordingly.
It's interesting to see how the model fails at extreme values. I can see why they chose the cutoffs they did!