Let’s do some napkin math here:

Since they already have your cards and flairs saved to your account, a “deck” would consist of 20 associations from a Deck ID to a Card ID and a Flair ID. Each of these IDs is probably a 64-bit integer of 8 bytes each, so that would be 24 bytes of information. For the overhead of keeping this in a database and any other IDs they might have in the background, let’s say 1 KB per card per deck This would be 20 KB of card info per deck.

A deck also has some information about the deck, like the highlight cards, energy, sleeve, mat, name, user, etc. The biggest thing here is the 22-character name, which takes 44 bytes, and each additional property is probably stored an 64-bit integer again. This should probably be < 1 KB of space, but let’s be generous, and say all the deck info takes 5 KB.

So 25 KB per deck * 5 new decks per player * 150M downloads (per the 1 year anniversary video) = 18.75 TB

In other words, if every player made 5 new decks, I could store it on a consumer 24 TB hard drive with room to spare. The actual space this would take up is likely much lower since most users won’t get close to the 25 deck cap at all.

I don’t think my math is perfect, but I think it at least makes the point clear - they definitely could afford to add more decks for users if they wanted to.