I would assume the C152 would way out live the PA38. Even with the wing life extension the PA38 has no future, 1000 hours is barely two years for the average school.
The C152 will keep going until crashed and damaged beyond economical repair. They are currently so cheap to run and parts are so readily available that they make ideal trainers, as they always have. Perhaps with an obesity epidemic they will become less popular, unless people either willingly or unknowingly fly them over weight.
It's a shame, since the PA38 is in theory the perfect training aircraft, and I as far as I know the only civilian aircraft truely designed for the purpose of basic training. I wonder how many are still flying in the UK?