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.
Yeah, microcenter is kind of the place to go for computer building as far as brick and mortar stores go. And most of the staff there, at least at the one near me, are pretty knowledgeable and willing to talk shop and let you pick their brain a bit.
Also just kind of a fun place to walk around if you like electronics, all the weird adapters and gadgets that you know must exist but never actual see in stores are there. The one near me also has a bunch of 3d printers and other maker/tinkerer tools and gizmos.
And their service department is usually pretty great (although I don't know if their online appointment system actually works, anytime I've brought something in to them with an appointment they kind of act like online appointments don't exist)
I haven't used their PC building service myself, but I'm pretty sure you could pretty much tell them what you want to do with your computer and they'd build you something suitable.