I think part of it is also an implementation pressure. If you put your dev work into building a system around managing subscriptions and processing user payments, its less easy to flip a switch and start siphoning data to 3rd-parties when they come calling.
In theory, this would be visible to end-users as a halt in feature roll-outs, because the dev team has to pivot to building ad-tech.
I hope to god VCs don't get involved; if they do I'd be the first to bail. I'm hoping that the revenue model makes VC money allergic to them in general. Bootstrapping is preferred for this type of service (see also: Pinboard, sr.ht)
Yes, I'm aware that all the same arguments apply to DuckDuckGo, and that Google & Bing already explicitly do sell this stuff, but the stronger the promises, the more I demand to see proof.