I can see how renting more cards is good for players in general, but I disagree that it is good economy that cards are artificially scarce from bots scooping them up, leaving less for real players to have available. A lot of the "big picture think" is nice in a perfect world but isn't practical in a real setting through the lense of a bot owner who thinks like a hacker. No matter how much you nerf rewards, bots WILL be there and WILL profit. It only hurts real players.
In general I personally am not a fan of multiple accounts in principle. I would prefer devs to force one account per IP. Players will be mad but it's for the better, transfer your wealth to one account quit gaming the system like a bot. (Once again artificial scarcity and hoarding is not "good economy," having more real players playing your game is.)
I agree captchas are aids. There has to be a clever and not annoying way to verify humans that hasn't been thought of yet that. Smart people assemble!