What if there were paid services that promised no ads? Ever? Make it part of their policy?
That doesn't mean I wouldn't use your service (which looks cool). It just means that I would have a plan in place for what I'll do when you sell out ;)
As for Write.as, I spend copious time talking about why I built it [0] and making sure people know me personally, to address your exact point. Personally, I'm a simple dude who relishes putting more value on principles than a number in a bank account. And I'd rather have a tent in the woods than Mai Thais on the beach.
But we do have an open API [1] and one-click export to get your data out, so you can be sure :)