I wonder what would happen if Let's Encrypt started charging for their service AFTER HTTPS became compulsory. Seems like a great (but evil) business strategy. All these CAs could just start increasing their prices and we'd all be forced to pay.
If you understand human behavior, then you know that this WILL happen eventually.
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.
I don't know who has what legal remedies when a nonprofit acts inappropriately, but another observation is that most of Let's Encrypt's technology is developed in public.
If you needed to set up another ACME-compatible CA on the same model (which could then be a drop-in replacement compatible with the existing client base), it would be a lot less expensive (although it would require datacenter build-out, hiring an operations team, and a variety of PKI-specific stuff like key ceremonies, HSMs, cross-signing, CPS, and audits).
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.
If you understand human behavior, then you know that this WILL happen eventually.