As someone who knew Durov personally, I'd say that it's very unlikely that the UX will be ruined by ads, and even more unlikely that users will be manipulated and/or tracked in any way. If there's one thing he does well it's respecting his users. He knows what "we're earning enough to cover our expenses" means. The company is privately owned so no one will be pushing him into anything either.
Many popular Telegram channels already have ads in them, posted by their creators. My understanding is that Telegram will be taking control of this by acting as an intermediary, taking a cut and possibly deciding what goes through. Thinking of it, it makes sense, and doesn't alter the UX at all for the average user.