Ah, you mean the original "razor and blades" business model that ensures repeat customers.

(Yes, I'm aware that many people who use safety razors these days are not necessarily buying from brands that make both the razor and the blades, I am such a person myself, I'm somewhat joking on that)

But even in the realm of "buy it for life" items, you can still end up with repeat customers. Maybe you want a second razor for your travel toiletry bag, or to keep in your second bathroom. Maybe you just see one that looks cooler, or the handle is more ergonomic, or the way you change the blade seems more convenient.

And BIFL items still do sometimes get lost, stolen, given away, thrown out, or sometimes even broken and need to be replaced.

And unless the world's population starts shrinking, there will always be new shavers hitting puberty who will eventually need their own razor.

With a DNA test, unless you're questioning paternity or testing for specific genetic traits like cancer risk and such, once your parents have taken a test, you and your siblings don't really need to, you know what your parents are so you know what you are.

Companies that sell safety razors (or whatever buy-once product) probably also do lots of other stuff to diversify.