you gotta think positive at least it's not your oldest credit card like Credit One or First Premiere where if you close it you would lose AAoA and drop 10-20 points so are forced to pay $99/year. You can always downgrade after paying the AF next year.
if you close a credit card, regardless of Lender, you do not lose AAoA. The card normally will report up to or more than 10 years.
A few other considerations...
Delta miles don't expire. The card doesn't play a role in keeping them active.
Amex sometimes targets downgraded cards with an offer with a SUB to re-upgrade back to an AF card. I earned the SUB on EDP, downgraded to ED, and earned another SUB when going back to EDP.
Most people are limited to 4 Amex revolvers. You still have a lot of flexibility there.
I agree with others who say this card achieved exactly what you wanted, "foot in the door". If that's your stance then card selection wasn't you top priority in the first place so it really shouldn't matter now. For people who are serious about it they reseach a product before applying to see if it benefits them long term. I don't understand the "foot in the door" approach myself considering how often this results in what some would call a "bucketed" card.
And there's big difference between Delta and BCP so it appears to show a lack of understanding in what it is you're truly after. That said, you might have had plans for the 50K miles and then Covid happend so that I can understand. Although this has been a real thing since March, and have had reduced travel in general since before 3 months ago. So it still makes me wonder why, when there were many other products to choose from.
As a general rule going forward i think the old "foot in the door" approach is likely not going to be the best thing in the coming months, and should probably be avoided during this pandemic. I will say that I think we all at one time or another made a mistake in card choice for whatever reasons, so it can be viewed as a learning moment.
I suggest product changing the card to no fee Amex Delta Blue after annual fee posts next year.
Product changing removes the ability to ever get the sub on the card. I would much rather close the card than to close myself out of the opportunity to get the sub on the card.
The Blue's a 10k SUB most of the time- the SUB isn't nothing but it's not exactly OMG AMAZING (lots of no-AF cards out there that beat that SUB, including some from AMEX). Having the tradeline stay open long term to help age accounts may be worth passing up 10k Skymiles, especially since OP got 50k from the Gold. As mentioned, there may also be upgrade offers back to Gold eventually. It also means that if you're trying to not open a bunch of accounts to achieve 5/24, you don't have to close Gold/open Blue and take two 5/24 strikes, as you've only taken one.