Glad to hear your apricots/plums are holding up well; I hope they continue to do well.
I saw on this old Houzz thread that two respected growers have had issues with Mustang: Bakers Gold Apricot - NW Calgary
I think the apricots they are referencing are P. armeniaca x P. mandichurica hybrids, which also have compatibility issues with Krymst 1. I assume yours are pure armeniaca?
Here on GF a thread reported mixed compatibility with Euro plums, but compatibility with an apricot: Mustang chum graft compatibility
Longevity usually is a price paid for precocity. Two of mustangs’ parents are short lived: P. Besseyl (10-20 years) and P. Americana (15-20 years). The above forum thread notes disease issues, which also shorten lifespans. For Zone 3 it probably doesn’t make much difference, but for Z4+ there are definitely longer-lived options.