An RF mount APS-C mirrorless makes sense for birders, but little else. Existing mirrorless APS-C cameras sell for only a little less than their FF counterparts, and aren't much smaller or lighter when using mounts common to FF.
Catering to the birders is fine, but there exists very little advantage to APS-C in terms of size or weight when utilizing a FF mount. After having and using an RP for the last year, I wouldn't consider going back to an APS-C. Cost and weight savings would be minimal, and it would represent a step back for most uses. Saddle an APS-C camera with a FF mount and you sacrifice most of the potential advantages of a camera with the smaller sensor.