You're right. This is a trust issue that's difficult to reason about. I'd like to encourage people to self-host as much as possible and I plan to be as open as possible about how I'll do hosting. Paid hosting means the revenue model is clear--there isn't a need to sell data to keep the lights on. I'm also using the same AWS deployment process for paid hosting as I make available in the open source repository--so every Haven gets its own dedicated EC2 instance, with a database installed right on that instance. This makes broad querying of everybody's data much harder.
I think my biggest fear is that that this idea takes off, but some competitor makes a free offering by selling data and including ad targeting and most users don't know the difference.
I don’t think you have anything to fear. The reason people would buy is privacy. If someone gives it away for free they are the product and their privacy is at risk.
Plus the market is enormous. Probably room for a handful of good sized competitors.
I think my biggest fear is that that this idea takes off, but some competitor makes a free offering by selling data and including ad targeting and most users don't know the difference.