It would destroy the current whale business model, but it might still result in going after big spenders in other fashions by getting more games for them to play a shorter time each and it might result in them modifying the feedback loop to get more people spending the maximum allowed.
Also, are the minnows really loss leaders or just not as profitable?
The business model for these games is that 99% of players spend negligible amounts, and 1% spend hundreds or thousands; the minnows are a loss leader to get to the whales. Restricting spending to $50 or $100/month would destroy games like this entirely. (Which I'm fine with, personally.)