Wimbledon is (in railway terms), not that old.
It was built on site of the Durnsford Road power station which had been built for the initial electrification schemes
Wimbledon Park is older, and not fully used nowadays as most of its sidings are limited in length (the site is roughly triangular), the ones nearest Durnsford Road can only take 4 or 5 cars. These are not going to be of any use when 10car fixed formation is the norm.
I‘m not sure if removing some sidings would help to get more 10-car ones (and there are 3 sidings parallel with the Wimbledon Park Line) Ideally need to take over the site of Industrial building in Durnsford Road, extend all the sidings to 12 car, and raft over whole site. Would be much more useful as undercover stabling / cleaning location (similar to the Central line sidings under Westfield).
I think part of the indecision is a proposal to use part as a crossrail 2 worksite / compound (if that ever happens)