sorry but a whole system which use 600w is using 50 amps so won't deliver hundreds
...that's at 12V. Amperage is relative to voltage, after all. CPU VRMs deliver anything from ~.5V (idle) to ~1.5V (load). That translates to >100A for a high end CPU at stock clocks between the VRM and CPU, and several hundred for something really power hungry like a clocked-to-the-nines 9900KS. It's true that the amperage between the PSU and VRM is much lower, but again, that's because the voltage is higher. This is the whole point of moving the VRM as close as possible to the power sink, after all - the further away, the more overbuilt cabling/traces you will need.
Wouldn't something like this make it harder to overclock or would it make it easier?
It wouldn't change anything in that regard whatsoever. CPUs already run off the 12V (only) EPS connector. It will likely make better 12V PSUs cheaper though, so as such it
might make overclocking ever so slightly "easier" to achieve - but that's such a small difference it likely won't be noticeable.
Nope... some even modern drives during spin-up eat over 3A... not the greatest idea.
Couldn't that be solved by spinning up the drive more slowly? I guess that would require some firmware trickery, but I can't imagine that it wouldn't be possible. But as I stated above, adding a 12V3A mode to current USB-C ports and front panel USB-C ports should be relatively trivial, and that would be sufficient for what you're describing. It's not like spin-up is a sustained state, and delivering the max spec for a few seconds should be fine for any properly made connector.
relying on motherboard makers to make great VRMs to deal with this is going to be a disaster, at anything but high end
That's what standards are for. The current ATX power supply standard specifies pretty much every single relevant specification for a PSU to be compliant. There's no reason whatsoever to expect a standard like this to be less strict. And enforcing minimum MTBFs or similar specs for the relevant components would be a simple addition to the spec.
Beyond that, why, exactly? Other than for overclocking, can you show me an example of a motherboard with a CPU VRM bad enough to actually be problematic?