"What of folk with partners or families for whom 15 miles is currently hard work and who don't feel safe on the roads?"
Thinking primarily of going for rides with partners and families.
50k of fairly flat is still an extra 50% of the ride you've just done. My first audax was a mere 60k, starting at Dalmeny and only 10 miles there and back (20 total) from my house at the time - I was absolutely wrecked after 60k. Now, as you say, I wouldn't give it a second thought really for that distance. But if I was to be riding a 200k (say) I'd consider driving the bike a similar distance because that's probably at the upper end of my ability.
So yeah, depends on the individual, the ride, the distance and terrain toa nd from the start, the accompanying party, etc etc etc.