I too have been protected all the time on alternate flights without any controversy. Make sure you’re asking for Delta-operated flights, of course, and that the alternatives are rational for your situation. You do typically need some sort of justification for why you want the protection, but legitimate delays are a perfectly good reason.
Oversale shouldn’t play into this - the amount DL is willing to oversell a flight is already baked into the availability in the GDS. If an alternate flight is Y0 and there are no seats, though, you won’t be able to get protected on it - and I don’t think there’s a “standby protection” process as it does require a confirmable seat.
Perhaps the problem you’re running into, RealHJ, is that you’re flying some cut rate budget airline called “Dullta”? I’ve never heard of them but I can’t imagine they operate flights with the same reliability and customer service as Delta
*ducks*