There was P2P chat application called FireChat - https://en.wikipedia.org/wiki/FireChat which used end-to-end encryption and so I think man in the middle attacks shouldn't be a problem. Considering decentralized platforms like Matrix, Mastodon are well established now , building a decentralized dating platform on top of those might be the rational choice.

You're right about the monetization, But I think the need for privacy has gained enough momentum now to sustain the business if it is able to deliver what people want.