I would think that there are enough competing vendors, and they are sufficiently interchangeable, that one vendor having low prices will drag the whole market down. That is, I believe that CAs are actually a nearly efficient market.

This might even make sense as "a great (but evil) business strategy" except Let's Encrypt isn't a business, it's provided by a charity, ISRG, the Internet Security Research Group, set up for exactly this purpose by people from Mozilla (a charity) and the EFF (another charity)

I suspect that the people behind ISRG weren't as paranoid as the Free Software Foundation about being corrupted by some hypothetical evildoers (the FSF has a whole mechanism to try to ensure that if you somehow take over the Foundation you can't use its resources to counter its original purpose) but you're going to need a bit more than a vague idea that people are capable of evil as an explanation for why good things are actually not good.