Consider Magpul[0]. They built their company selling a what basically amounts to a thick rubber band with an extra loop on one side[1].
Over the course of the following seven years, they landed defense contracts and made enough profit on that little doodad to design and manufacture the PMAG[2], which is their best-known product now - and I strongly suspect, their primary profit driver. They've sold absurd numbers of PMAGs over the years.
After that, they were a well-known name in their target demographic, and were able to parlay that into a line of "furniture" for ARs - stocks, hand guards, grips, etc. These days they sell everything from rifle magazines and accessories to wallets and iPhone cases. They come to mind immediately for me because I saw yesterday that they'd launched a line of safety glasses[3]... that start at ~$150.
Magpul's product line is the very definition of "durable goods", but they've managed to build a thriving business around them. The keys to that as best I can tell are: never stop building your brand, and never stop researching and releasing new products.
0: https://en.wikipedia.org/wiki/Magpul_Industries
1: https://gunxtras.com/image/cache/data/MAGPUL-Original-Magpul...
That said, just like clothes, the products will need to evolve or expand if the company is to see long term success.
They sold to Unilever in 2000, after 20 years in business.
Also maybe you can sell out to the big guys.