How do you know they didn’t use new parts?
I don't. But the bear bomber is very long in the tooth, and I don't think any new ones have rolled off a Tupolev production line in quite a few years. If they are new parts, they are left over from previous production runs.
Besides, canning old birds to keep others flying is a time honored military aviation practice (there as well as here). It is the most cost effective way to keep older aircraft flying.