How serviceable are watches with defunct manufacturers? My understanding is typical mechanical watches require service to replace worn parts every 5 years or so.
Most services just include cleaning and relubrication. Parts don’t wear out that quickly.
Most watches from before the quartz crisis (pre ‘70s) used standard off-the-shelf Swiss movements for which spare parts are still readily available. It can get tricky with expensive or rare in-house stuff like Omega and Rolex though.
For vintage (ignore their new stuff), Enicar, Eterna, Roamer, Rodania, Cortebert, Wakmann, Mido, Certina, early Girard Perregaux, etc. all can be had cheap. Bunch of excellent brands wiped out in quartz crisis that can be had for next to nothing.