I think one thing to keep in mind with business rewards is just because DL puts a price on a business seat of $4k doesn't mean the seat is worth $4k to me.
Put another way, if I would rather have $2k in gift cards than a business seat that retails for $4k, then the value of the miles is the $2k in gift cards. The list price of something I'd never buy doesn't determine the value of miles I'd never spend that way, because I'm not using the miles to save me $4k. I'm using the miles to save me $2k.