No individual card text (limited to just the mechanics) is copyrightable but the setlist of cards might be. It would come down to how much creativity went into curating the list of cards that is released. It gets especially murky because new cards are always being released and old cards are being retired, so they obviously put a lot of creative energy into that process. You'd have to avoid pre-made decks as well.

Unless you have funding from an eccentric MTG-loving billionaire, I see why you'd comply with the cease-and-desist.

Hasbro had the legal president too, as they were involved in the Scrabble lawsuit, which I think is mostly where the concept of not being able to use patent law for game rules, but did set the trend on aggressive trademark interpretation.

I expect the genie is mostly out of the bottle at this point. I'm fairly confident that people can do X and Y actual illegal things on the Internet, we can have our card game, but I hope it can happen with a site or decentralized system easier than doing on Tor.