The parts last quite some time if properly maintained. If you're worried about replacement parts availability, stick with the most popular movements such as the ETA2824/SW200 series or Seiko's NH35 series.
NH35 can be serviced, but makes more sense an assembled replacement part. The same argument can probably made for the entire watch. The appeal of using one watch for 30 years is more in romantic fantasy than practicality.
That said, a lot of in-house movements are little more than tweaks and high end finishing applied to existing commodity movements.