But we have already seen it happen with the telecoms, so while they may not be "rational actors" clearly it's not too small a market oportunity to be worth it for some spineless businesses.
Telecoms also generate their revenue by just keeping the power turned on with their existing infrastructure (yes it does require maintenance and monitoring), but that didn't stop them from trying to find additional revenue streams by further exploiting their already paying costumers.
I do use Cloudflare on their free tier, and I don't think they are spying on everyone and selling ads against some profile they have built up. We'll just have to be vigilant because the status quo right now is to be big brother and sell targeted advertising.