Comment by JohnBooty

Sadly, I'm not aware of any specific "we will continue to manufacture parts for X years" guarantees from Seiko.

Still, though - keep in mind that entire Seiko movement can be bought for like $50-75. And many of their most common movements are interchangeable. For example, an older 7S26 can be replaced with a new NH36 that sells for about $50. https://chronometercheck.com/seiko-7s26-movement/

It's not entirely unfeasible to imagine buying a spare movement or three if you were, say, planning on doing sort of a heirloom thing and wanted to ensure a supply of parts N decades into the future.