Why hasn't anyone thought of this before??
Well, to be fair if you want repeat donations, you would think twice before being too obvious about double crossing the donor.
But I also think the rich donors are more stupid than they seem from afar, so smooth talking them is probably not that hard. Nowadays there's a lot of them anyway lol
Moreover, ultimately rich donors are still human susceptible to persuasion. It's not like you can't possibly present an argument for your desired policies that seems like it's in a donor's favor, or something like that. I've seen people rag on George Soros here before as if he were some part of a conspiracy (probably new leftie to politics reading right wing material lol), when he's one of the few very open billionaires supporting liberal causes.
Sometimes donations don't actually serve as "hey, we want this loophole" and is more like a ticket for access to the candidate to try and make the case for the loophole.
People overemphasize the money, and in hating that they end up thinking of money as everything. If that were the case, then Sanders and Bush would have won. Sanders certainly thought that the more money he threw at the problem, the more likely he would fix it.