If someone needs cheap but good cable, https://www.blitzwolf.com/5.9ft/1.8m-PVC-2A-Micro-USB-Charging-Data-Cable-p-283.html
It's great to have actual numbers available. 22AWG (0.33mm^2) wire has resistance of 53mOhm/m; hence, a 1.8m cable has a typical resistance of 190mOhm, so +/-20% manufacturing tolerance assumed, let's say worst case is around 230mOhm. Combined with the max contact resistance (e.g., for a Hirose microUSB,
http://www.symmetron.ru/suppliers/hirose/files/pdf/hirose/hirose_ZX.pdf , 30mOhm) - two contacts at both ends - we are at 350mOhm.
So, given my earlier exemplatory numbers (from the top of my head, to illustrate a concept, or point) equaling to 500mOhm total resistance you called "nonsense", they were not very far off from your specimen of a "good" cable of 350mOhm!
So, using this "good" cable alone will cause a 700mV voltage drop on my claimed 2A peaks (which should be actually verified), browning the power out to 4.3V. A short 4.3V valley is still probably OK for the Raspberry - this would need to be measured as well -, but even if it works, I know for sure there's not much margin left.
Now, as for the smartphone comparison, a charger buck converter IC might typically have maximum duty cycle somewhere in the 90-95% range, and power stage losses included, maximum output for 4.3V input would be around 3.9V, which would equal about 50% SoC while charging. This agrees with what the people are often actually seeing: poor cables charge very slowly (sub 1A), while "good" cables such as this charge at about full rated current (1.8A here) to an emptyish battery, but tend to taper out somewhat. This is often acceptable: we want fastest charging to fill the emptyish battery.
It's worth noting it's a normal trick for a 5V USB supply to compensate the output voltage assuming a certain wire resistance. Such a supply might also help on the Raspberry, but the response time of such compensation may not be enough; this feature is designed for phone charging, after all. Many 5V chargers also output 5.1 to 5.3V. This would probably "mask" the resistance of the cable so that the "full" 1.8A charging current is seen over most of the cell state-of-charge range.
So the user of this cable would be completely happy using it for charging.
For powering a Raspberry, I'd expect, given the information available and my design experience, that it's borderline failing. It's
likely to be OK, but the margin would be poor. So either the user would be happy; or the user would experience very weird, random and possibly rare reliability problems. Now changing to a different "good" cable could either fix it for good - for example, if the better cable used 20 AWG wires, or is shorter - or then it just borderline fixes the issue by being just marginally (say 10%) better. Or, the "new" better cable could just be worse.
When it comes to reliability, this number game is getting
really nasty. Simply put, way too many variables, way too little margin, way too much uncertainty.
I'm not at all surprised at your earlier comment:
"when people had power/reboot issues, the first thing I suggested was to change cable/PSU and it solved most of the problems"
But change to what? It's
clear that only the very best supplies and cables could give a good reliability margin. With medium-class stuff people would be most likely to find, even when shopping for "good", the devil's fully in the small little details and environmental variables with poor margin for error.
Now, the OP's problem required a 8' (2.4 meter) cable. If 22AWG wire was barely working / on the brink of failing at 1.8 meters, at 2.4m it won't cut it anymore. I'd expect the available market options for a 2.4m microUSB cable with at least 20AWG power wires, is going to be harder and fairly expensive.
OTOH, if powering through the GPIO header pins (30mOhm max is BTW also typical for said connector type; but they have paralleled two to halve the effect of contact resistance!), it's utterly trivial to just use your go-to 16 AWG or 1.0mm^2 or whatever wire you are using for power in your projects, and easily get the total down to less than 100mOhm, even with long wires up to several meters. No issue whatsoever! Only after about 3-4 meters, you start worrying about the copper cost and the heavy cables, and need to start thinking about power distribution with higher voltage, and local buck regulation.
It's also worth noting that the assumption that you "shouldn't" consider connecting anything to the Raspberry is unrealistic, and serves no purpose (except to keep discussion going). It's a computer with USB and Ethernet connectivity, of course people are going to connect things to it, such as keyboards, mice, 4G/LTE modems, or external wi-fi because the Raspberry doesn't have a connector for an external antenna. While it wouldn't make much sense to calculate for the absolute worst case of loads, it's not acceptable to completely ignore very typical loads either.
This being said, by all means do ignore the real-world usage patterns, and you are guaranteed to see problems, and you are guaranteed a job of fixing issues, or have endless discussions