$550 AF just posted on my converted Amex MB Plat. I was thinking I'd keep it anyway since I want to stay on Amex's nice list, and between the airline incidentals, Uber, and Saks credits, and Amex Offers, this card is basically a wash for me. I'll have my Biz Plat for Centurion Lounge access until at least August or September. But after that, I don't want to churn through another personal Platinum (i.e. Ameriprise, Charles Schwab, or Morgan Stanley) yet because I'm trying to stay under 5/24 until later this year to pick up a few more cards from Chase. Another two cards won't drop off my */24 count until April 2020, so I need to slow my roll.
I called into Amex's retention department (1-800-452-3945). The rep looked up my account, noticed my spend on the Plat was only "average"* and I have "all these other cards, the Gold, the Biz Plat, a Biz Green, all the Bonvoys, why would [ I] want to keep the Plat?"

So tables turned, I list the things I value about the Plat, but tell him that practically speaking, for most things I'm spending on every day, the Plat is a 1x MR card, whereas the other Amex cards in my wallet offer more rewarding bonus categories and I'll put my spend where I get a better bonus. The rep says he'll see if they can offer me some sort of spending bonus that'll help me put more spend on the Plat, puts me on hold for a minute, and comes back with
30k MR for $4k spend in 3 months. Done. Took the offer.
*ETA total spend in the past year was only about ~$11.5k. The rep did say I did "pretty good" a few months ago, which I'm guessing refers to over the holidays and when we bought our Casper mattress, when the statement balances were in the thousands vs. a few hundreds usually. Amex obviously cares about spend.