I did it on the Delta Gold Biz when it had an elevated offer. The offer at that time was 50k SkyMiles + $50 credit towards a Delta purchase for $2,000 spend (and the AF is waived in year 1). I sold the $50 Delta card on Raise and received $42 and got $250 in Target Cards from the SkyMiles. (I fully acknowledge there was more value in using them for flights but I had no reason to ever fly Delta so that was off the table). $292 back on $2,000 at a 14.6% return was good enough for me

I know Delta offers change all the time but right now I think it's only 30k miles + $50 gift for $1k spend. I'd probably hold out for a higher offer. Of course the other thing now is with AMEX, signing up for a card for only the bonus isn't a great plan as it can get you in their doghouse. I'd struggle to continue to put spend on a Delta card at a terrible return just to keep them happy...so that warrants consideration too.
I'm not too far behind you on this...it feels super sad...I agree