Springs wear out like everything else. It is nowhere near impossible, it is something that will happen with enough time and use. Especially springs subject to vibration and heat and who knows how many cycles. Want me to send you my old valve springs for your car?
Also, inadequate volume also means low pressure under high demand, it does appreciably affect pressure in the real world. Two different things, but you need both or the result is the same. Too low on the pressure equals low volume because it can't get enough fuel thru the hose, too low volume equals low pressure because demand outruns supply and the pressure drops.

It is easy enough to check fuel pressure, then whatever may be wrong can be worked out. It is not normal for one car to vapour lock while similar cars with the same setup do not.

Sure. But this particular spring doesn't generally wear to that extent.
Nope. I want you to send me a fuel pump that has worn out in the manner you describe. (Please send it by mail, not UPS. )
Definitely in theory. But in practice, in the case of a slant-6 engine with a 1bbl carburetor, it doesn't — not with the tiny orifice in the carburetor inlet needle seat being the limiting factor.
Agreed!

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Too many people who were born on third base actually believe they've hit a triple.

Dan, I got rid of that pump when I replaced it. I don't have room for usable parts, let alone junk!

Just for fun, check the pressure on some older pumps. It will usually be less than it was when new.