I think POTS is going to be around for many more years to come. In my immediate area demand has actually been increasing in the last few years due to new housing developments and people turning what were once holiday homes into permanent residences. If you walk around my little settlement you'll see dozens of DACS units on poles (multiplexers to put two or more lines on one physical pair).
Being out in the boondocks (by English standards), cellphone coverage can also be fairly patchy here. Vodafone is pretty good, but the others still have quite a way to go to fill in the gaps.
There's also the problem of the cost of calling a cellphone from a normal landline. We operate on the "caller pays" principle here, unlike the U.S. where calling a cellphone costs no more than a call to a regular line.
Calling a cellphone here is much more expensive than calling another POTS line, and many people would still be very reluctant to call if you could only provide them with a cellphone number.
