The reasoning in this thread started with something along the lines of "if you use your computer professionally... [statements]". I would argue that if you use your computer professionally you should definitely consider using specialised products, which includes custom built desktops as you correctly noted.
1 I would have got the person at the shop check it and tell me if I need a new laptop or a new part, this happened to a laptop of mine and I decided I would get a new laptop, then the shop person offered to buy the display and RAM from me and I sold it, so a different person would be able to fix his laptop
2 a spare part can be found from a different laptop or ordered online by the shop, so they can order it at the time I arrived with my broken laptop
I don't think that I would have been screwed but let me know if you still disagree.
Equally true for Apple hardware, that you can get in most reasonably sized shopping malls basically anywhere over here (France). Most places that sell laptops also sell Apple hardware.
In that case where there is no Apple sanctioned repair presence such shops either usually sell Apple hardware and gladly perform support and diagnosis if that can bring them a sale, or they're nice enough to help you anyway with the diagnosis out of good will (people are generally like that in digitally scarce areas) and will direct you to get a new one at the mall (or, they're dicks and then, screw them).
You have every right to call me an optimist about human behaviour though ;)
Granted you’d be equally screwed with a Mac Mini or iMac but fundamentally this has nothing to do with Apple vs PC. Desktop PCs that can be changed with standard parts or even custom built are not the market norm anymore and entered the specialised market realm long ago.
Your post also reads very much as "You're wrong to use Apple" but you have to realise that they produce a very reasonable solution for many. My use case covers a wide spectrum of mobility, security, dev, music, and photo/video editing, and while alternatives can do things like that they won't do so as efficiently and with the same quality across that whole spectrum for me. It would be foolish to shoot myself in the foot by nerfing my workflows every single minute of the day for a rare event. I do have a spare machine (which does not need to be the latest and greatest so can come out pretty cheap), which allows me a very quick turnover to continue working, well within an hour of a fatal failure (and that's even though I have an Apple Store 5 min away from my place).