Unless a contender has a serious goaltending situation (injury or underperformance) and creates demand, Grubauer will be here until next off-season. His RFA status won't impact his value, as the other team would still retain team control of the player. RFA rights are substantially more valuable than UFA rights (Darling).
Plus, if you're operating under the assumption that he gets a Darling-like contract, then the compensatory picks are a 1st and 3rd round pick for anything over $3.925M. A 1st and a 3rd is a good return. Offer sheets aren't really a threat here. If they sign him to something the Caps don't want to match, then the Caps get a good return. If they sign him cheap, the Caps match the offer and then trade their affordably-signed 1B goaltender to the highest bidder.