Ah, the wiki. Yes. But the wiki is wrong, hence I don't read it
E.g. I quote: The spinning reserve is the extra generating capacity that is available by increasing the power output of generators that are already connected to the power system. For most generators, this increase in power output is achieved by increasing the torque applied to the turbine's rotor.
That is not a spinning reserve. That is a simple load following plant.
So, the primary argument that a solar/wind based grid needs more of those 'spinning reserves' can only be considered true if you had a lot of mid range load following plants running with "reserves". (You implied we would need to build new ones)
Which would contradict the fact that we already have a three phase (primary, secondary, tertiary) reserve energy supply.
The PDF explained pretty clear what kind of reserve energies exist. And the name 'reserve' has a specific meaning, it does not mean what laymen think it means. I really doubt a country as advanced as the USA still has "spinning reserves" (in the classical sense, not in the sense the wiki implies). Germany definitely has not!
No, my country doesn't require 'undisrupted power supply' because that's a standard they can't meet.
The 'undisrupted power supply' is not the point. the point is getting back to power quickly and even more important: avoiding that power disruptions happen.
In Europe this is regulated. As I said, no idea about the USA, but I would wonder if it was not.
In other words if a power company plans to feed X GW into the grid, it is required by law to "back that up" with enough reserve power (the way how to do that and which amount fitting to X, is defined in the law).