If you expect income, you're not really looking to build a free product, are you? Seems to me, a paywall (or similar) is the honest way; those who get value from your product give value back, in the most direct and transparent way possible.

(I know I'm trivializing the question; I'm not in your situation and I don't know the perfect model for your product. But I disagree with just lumping together "paywalls" and "advertising" in the same category of "non-desirable things".)