Great in theory but if you are managing a system now, you would rather spend money on things you need now, you aren't going to keep it in the bank for some future day when the escalator needs replacing.
I suspect that equipment is depreciated in TfL accounts but the people 50 years ago wouldn't know how much this replacement would have cost so who knows.
The trains will run for 100+ years. This is possible because almost all parts can be made in house and those that can't are eventually replaced.
I'm fact the Zürich rail company (ZVV) in their purchase deal of the bombadier light rail included all plans for all parts. Bombadier was not happy but the ZVV has to make sure they can service the trains even if the manufacturer goes away.
This is also the reason LED lighting it a very delayed roll out in the city because the city can not use a light that is vendor locked which most commercial LED setups are. The old Natrium lighting can be purchased anywhere from. many different vendors. The fitting is standardized.
I suspect that equipment is depreciated in TfL accounts but the people 50 years ago wouldn't know how much this replacement would have cost so who knows.