Pilots already know how convert between magnetic and geographical headings, I would think. I think the small inconvenience is better than having outdated runway markings or having to renew them every now and then (not only on the runways themselves, but also on all charts).
Um, no. First of all, the conversion isn't consistent from place to place. For example, as you move across degrees of longitude, the angular difference between the magnetic and geographic north changes. So, your conversion changes. The last thing you want to do to a pilot is to add more shit on his plate, trying to do calculations when he could simply compare his compass and the giant number printed on the runway.
FWIW, I was a private pilot in the 80s & 90s, but gave it up when my kid was born...just didn't have time.