Comment by nancyminusone

It's done when there is no need to improve it anymore. But you can still want to improve it.

A can opener from 100 years ago will open today's cans just fine. Yes, enthusiasts still make improvements; you can design ones that open cans easier, or ones that are cheaper to make (especially if you're in the business of making can openers).

But the main function (opening cans) has not changed.