Oh wow, what a bummer, didn't hear anything about this neither. In any case communicating with Amex has been very challenging, especially on bonus points and other perks. They were never able to respond, still waiting for my bonus points. This forum is a better place to get info :-).
One thing I liked about Amex was the 'no spend limit' , sometimes it can get really crazy and in those cases it's good to have no linit on spend. With Mastercard you need open a creditline, i.e. they check your finances, just like with any loan. Your Amex card doesn't even show up in active loans. I noticed it on the paperwork when signing up for the Mastercard, no trace of the existing Amex.

The really 'old' Brussels Airlines frequent flyers may remember there was once a Mastercard, BCC Corporate, or something. The acceptance of Mastercard is in any case much better. Downside is going to be the 100.000 miles / year cap. With the old Amex I was able to rack up a huge amount amount of miles. Never even tried converting the membership rewards points. I do like to have 2 options in my pocket. So likely going to downgrade to a cheaper Amex.

Is there a forum thread somewhere discussing the June changes / devaluation?