I hesitated to weigh in because the answer is very dependent on the rider and the trails where it will be used, but... I wuill say that I have ridden a lot of pretty gnarly single track on my old rigid 1990 Canondale and still ride it some of the time. For some terrain it is still a great choice and it wouldn't be out of the question as an only bike.

Many of my closest to home daily rides are just chock full of real sharp edged bumps (roots). They are the constant stutter bump kind of trails with bigger ones on the little sharp climbs and descents. The rigid fork can be a bit jarring to my arthritic wrists there so I use my bike with a front suspension. If the rigid canondale had really fat tires it would help though.

I sometimes drive to some trails that are less jarring and the rigid canondale is a super nice ride there.

Fitting up the fattest tires that would fit on the rigid canondale would level the playing field between the two on the choice of which to ride where.