I did a fairly thorough analysis of the cards I have and some potential new ones for brands I use but don't have the card (United and Bonvoy).
For now, I'm keeping the DL Plat primarily for the $2500k MQM head start, not sure I'd even get Silver without it as it's hard for me to use DL on my international business travel given the pricing, but also the mileage multipliers are pretty good. It's a bit better on Grocery/Dining with the HHonors Surpass which I have, so I'm prioritizing spending there first until I hit the $15k for the free night cert, and then will move that spend to DL. ANd of course the 3x hotel charges for anything I book direct (e.g., not via Amex Plat prepaid which gets 5x MR).
Companion cert - eh, we've used it once or twice, but often the booking classes aren't available, or we're flying separately in one direction, or some other reason.
The 15% off redemption is obviously great, just saved 33k on a DeltaOne to Europe (220k->187k)!

But I'm resigned to dropping from Plat to probably Silver, maybe Gold, will miss C+ at booking, I suppose I'll have to slum in exit rows or just pay up, probably to F rather than C+ though.

I ultimately decided against any UA/Bonvoy cards, the return on spend wasn't particularly compelling (personal airfare goes on Amex Plat for 5x), and neither were any of the other perks of either. I mean the PQP earnings on the UA cards are worse than the DL MQM on the DL Plat!