Earlier quoted context omitted.
A mechanical watch needs regular service which is usually just cleaning, lubricating, replacing seals and springs. Eventually it will need replacement parts and that is probably the end of life for the watch. There are a some brands that will service every watch they've ever made, fabricating new parts as needed. Few of us can afford one of those.
I have seiko automatic which kept great time when I first bought it, but not long after, it slipped off my wrist and fell on the tile floor. Since then it has been losing time but no so much as to be a huge problem. Would service easily fix this or is it just something to live with?
That said, simply adjusting the tick rate to regulate timekeeping is very easy and if you've got steady hands and a sharp eye then you can do it yourself with a wooden toothpick, assuming you've got a tool to remove the watch back. Any shop (offering repair facilities) could do it as well in a matter of minutes.