The situation with the 707s is astonishing, the apparent net waste could have reopened a couple of old branch lines. The 1st January 2020 deadline for DDA compliance has been on the horizon for a long time, yet we have 30 Class 321s being refurbished even though they'll be lucky to run in service much beyond 2019, traction package upgrades and a whole fleet of DDA compliant trains with no home.
It's a shambles and it all rests at the door of the Department for Transport. But this short-termism is not exclusive to that one Government department, so you'll be certain that similar wastes of money go on in other service industries.
Yes, the Class 411's lasted a long, long time, a handful of 309s saw further service in the North West until 2001ish but they were in a poor state by the end. The relative cheap cost of leasing new trains is certainly a factor
for the situation but so is the relatively lower running and maintenance costs of owning a fleet of new trains, then there's the lower financing costs, DDA compliance, better braking and acceleration and finally (Thameslink commuters might want to look away now) there's the "wow" factor of having a new fleet.
Those holding out hope that the Networkers (north and south of the river) and Mk3 based EMUs will be around well into the 2020s should be well advised to buy a Network Card and enjoy them now.