I'm curious, if you're comfortable sharing, what was your spending profile on the card? How long have you been a Silver cardholder?

I had something similar happen when the Hilton portfolio moved from Citi to Amex 8 years ago. I received a letter saying that my Hilton Honors Reserve card was converting to a ThankYou Preferred card when everyone should have been moved over to Amex. I was (and am) in good standing with both issuers, so I don't know why my card didn't move over. I elected to close it prior to the product change, and I opened an Aspire card when Amex opened applications for that in early 2018. I didn't see data points from others with similar stories.

The Arrival+ isn't a bad card for 2x earning for non-category spend, but you'd probably be charging travel purchases to another card with a category bonus. It's also interesting that they're product changing you to that card since it's not open to new applicants and hasn't been for some time. While there are some transfer partners available now, the transfer rates aren't that good.