How would recipients know to expect an S/MIME signature though. It's not like it's enforced by MTAs like DMARC is.

IIRC, if you're using Apple's Mail client it gets validated against the root cert shipped with MacOS/iOS. You get a little black tick next to the sender.

In theory, third-party places like gmail could (should ?) automagically verify S/MIME sigs where a root cert is readily available.