That voltage stability is electromagnetically coupled to huge chunks of spinning iron. That's your inertia. It buffers voltage spikes. Solar, and surprisingly enough wind as well, do not contribute to this stability. It's one of the big challenges we need to figure out.
Actually, this is something I'm not clear on: Why can't we just add free-spinning "generators" to the grid to contribute inertia?
Costs money. And, unnecessary.
It's pretty clearly necessary to solve. And any kind of energy generation costs money. So I don't think that, unqualified, those are good reasons.
Free-spinning wheels generate no energy. Electronics do the job much cheaper.
Wait what, Newton's inertia was about physical objects, not voltage stability? You talking base vs peak demand or what?