That is incorrect. If there is no chance to get to your destination in a reasonable time/schedule that is needed then DL will give downgrade compensation as you paid for a higher class but are flying in a lower class seat. Especially if the delay and subsequent re-routing/rebooking is due to a DL issue (crew, mechanical etc). Saying that you have to wait for potentially days for your original booked class or sit in the back with no recourse when you paid more is not acceptable.

Maybe DL gets away with it on unknowing fliers who don't know to ask, but I've never experienced the situation where they said I'm owed nothing because I am taking a different flight.