Interesting point. Don't forget that this is 40-50 year old Soviet technology :). And cars are actually in pretty bad shape, well over their planned lifetime of 30 years (AFAIK). Full of rust, sometimes catch fire. The drive system is also problematic, because it doesn't have regenerative breaking so the cars heat the tunnels quite a lot which is pretty bad during the summer.
They are in such a bad shape and/or hard to rebuild that not much remains of the original during the refurbishment.
Actually the Russian company and the tender has received attacks that the cars are actually new, only some identifiers have been transferred from the old cars, as Metrowagonmash had a dozen or so surplus cars of the type the used cars were supposed to be upgraded to.
They are in such a bad shape and/or hard to rebuild that not much remains of the original during the refurbishment.