The problem is that there are few people skilled enough to do roofing as it is, and so even if we say 30 years life for plastic, its still better than the 5-10 year life of badly done mortar in valleys, verges and ridges
Have a quick look at all the box houses on new estates from say the late 80's and see how many have not had work done on the roof .... work which should not have been done until 2040
Also, home owners/builders are not really bothered if the new roof repair is done in plastic or not as all they care about is the cost and that they may not even be in the house in a few years time, so they are not bothered about the lifespan - someone else can deal with that