That's all true, and I'm sure that there will be variant decks included in the book.You don't necessarily have to use those cards. You can add extra cards. Variant Decks are a thing, it would be easy enough to detail them as magic items, and include custom deck tables. The main reason decks aren't used much is because they can, and often do, completely disrupt a campaign. But a custom deck can include only things the DM is happy with and prepared for.
But still, 200-ish pages on variations, spinoffs and elaborations on a single magic item? It really does seem hyperspecialised. Fizban's seemed to me to struggle for content at times (I have no idea what Bigby's is going to be like) and they are about entire broad monster categories and cultures, that you can build an entire campaign around. Doing the same for a magic item? It's certainly not impossible, but personally I find it hard to believe that WotC, whose watchword is 'generic', would put out a book that's so niche. But of course, I personally found it hard to believe that WotC would put out a Ravenloft campaign setting, or that they'd re-do Dragonlance before Planescape or Dark Sun, or that they'd do Spelljammer at all, so what do I know?
I do hope they take the opportunity to bring back a slightly tightened up version of the card-based rogue subclass from the Runeterra stuff D&D Beyond put out a while back. The wording of the rules was pretty terrible, but it had some interesting ideas, and the gambler rogue archetype is a classic and one we haven't seen yet.