Why is there an assumption that the card will be phased out?

Equally likely is a non-exclusive relationship, as Hilton has with Citi and Amex.

Of course, Amex could win the bidding war with Chase and become the exclusive issuer for the combined SPG-Marriott.

The card would need to be changed if and when SPG is rolled into Marriott - but even that is speculation at this point. There is the (slight) possibility that they maintain SPG as a stand-alone program for some time.