If the Captain did become incapacitated, being able to safely handle an airliner in otherwise normal ops will be more useful than being competent in dealing with an emergency in an SEP.
What if the Captain too had come through the "never flew solo" training path, though was just further along. You now have two pilots who have likely never had to actually make a life and death decision for themselves - by themselves while flying. 'Never dealt with a real emergency.
These are pilots who have never had the sphincter factor in their flying. They may be book smart, and sim savvy, but I feel there's still the "this is the real world and you have to handle it" element missing from their experience.
I think I would be more comfortable with the average 1,500 hour Cherokee and Seneca pilot being dropped into right seat of the stricken 737 to land it, rather than the never flown solo 1,500 sim pilot. The "real" pilot will have learned the skills of staying cool during a real emergency, and prioritizing what needs to happen to simply get down safely.