I can't see any motive for the smarter, more ambitious party members to try to cull the dead wood. Why should they? Someone may get elected who is a real threat to the existing member's advancement. Never doubt that personal advancement is far more important than national advancement.
My local member was voted in in '96 and has never risen above the dizzy heights of Ministerial Appointments
Parliamentary Secretary to the Minister for Defence from 30.01.07 to 3.12.07.
but I am certain his leader would praise him as a "hard working local member, etc" but never good enough for even a minor ministry. In spite of this uninspiring performance his party has never suggested he step aside for a "better" candidate in the four elections he has won since.
He is a classic seat-warming back-bencher but I'm sure he won't stand again. He (and wife) is currently on a nine nation "educational tour". That's the "Lap of Honour" I believe.
