Iam in for no cap too. Because the days I can play a lot (a free time weekend day or vacation day or so), I will probably exceed 5 hours a day...
The problem is contests... The fastest speedrunner, or best PvP player or that kinda stuff will be hugely influenced by the amount of upgrades you have. If there would be a solution for that problem, then you could completely remove the cap.
(Also, PvP'ing is a problem, if the matchmaking system puts two persons of equel skill against eachother, but one with 100+lvl deck vs one with deck lvl 20-40... just results in unfair matches. This again could be fixed. Probably something like including upgrades into matchmaking, which on itself introduces yet another problem because it is splitting up the maybe already smaller PvP community)