The DM has started to wonder how metal can be that durable.
Show him this:
The Eiffel Tower is 130 years old and made of wrought iron which is far less durable than the steel your armor is made out of. The tower will last for as long as it is properly maintained, as will your armor.
This beautiful piece comes from the Staffordshire Hoard and believed to have been created in the 7th century or earlier. Of course, its made of gold which, unlike iron or steel, doesn't corrode and so requires much less maintenance. On the downside, its very soft and therefore much more susceptible to damage - it is also as structurally useful as wet spaghetti.
Metal is extremely durable. Properly cared for, there is no reason it cannot last effectively for ever.
Which brings us to what the PHB has to say about all of this (p.157):
Furthermore, expenses cover the cost of maintaining your equipment so you can be ready when adventure next calls.
Ultimately, you will have a Ship of Theseus or Grandfather's Axe problem:
This is my grandfather's axe: the same one he used all his life, that my father used all his, that I use now and my son will use in his turn. It's only had 17 new handles and 14 new heads but its the same axe through and through.