I think you're massively overestimating the number of small businesses that approach $4MM in expenses in a good calendar year, let alone the number that would spend $4MM during what may end up being the worst year financially since the 1930s. And that's without even considering whether or not their suppliers really want to pay swipe fees, or if they could get such a better price by using cash that the card is costing them too much to use.

That said, there's an easy fix, if UA wanted to do this: cap it to 50K lifetime miles earned per MileagePlus account, and only during 2020. That wouldn't be enough to cause big problems with dilution, but it would probably generate a few additional purchases on United Chase cards.

I still don't think it's a very good idea, but neither does it have to be a disaster.