That assumes that the fuel runs out after 15 years - which is much less likely with electric propulsion since it uses so little fuel for station keeping. The other components don't have an unlimited life, solar panels and batteries degrade, etc.
I suppose if Directv wanted to continue satellite service but D11 or D12 was running out of fuel maybe it would be a better deal to spend $100 million or whatever to give it five extra years of life than $400 million for a new satellite that gave them 20 years - since they wouldn't need 20 years. But in most cases you'd be better off launching a brand new satellite. They are always more efficient than what went before, originally you needed multiple satellites for a location like 101 now T16 will handle that all by itself (and probably still have capacity left over for some Ka transponders if Directv ever used those from 101) Keeping an old satellite going for a few more years would have a pretty limited market.
Much bigger market to be able to clean up the old retired satellites in the parking orbit a few hundred miles above GSO, and boost them down to be able to burn up in the atmosphere. The question there is who would pay for that? Its like finding someone to pay for cleaning up an old city dump and turn it into a public park. It is for the good of everyone, but trying to raise everyone's taxes to pay for it would be difficult - a lot of them would say "I'll never use that park, someone else can pay for it!"