The people you're replying to aren't talking about converting from AC to DC or stepping voltage up or down. Rather, they're talking about grid stability. You can have mechanisms to convert from AC to DC and to step voltage up or down, but still have a unstable grid. We had a notable example of that last year:
https://en.wikipedia.org/wiki/2025_Iberian_Peninsula_blackou....
One way to think about this problem is that our electrical grids are giant machines—in many ways, the largest machines that humanity has every constructed. The enormous machine of the grid is comprised of many smaller connected machines, and many of those have spinning loads with enormous mechanical inertia. Some of those spinning machines are generators (prime movers), and some are loads (like large electric motors at industrial facilities). All of those real, physical machines—in addition to other non-inertia generators and loads—are coupled together through the grid.
In the giant machine of the grid, electricity supply and demand have to be almost perfectly in sync, microsecond to microsecond. If they're not, the frequency of the grid changes. Abrupt changes in frequency translate into not only electrical/electronic problems for devices that assume 60 Hz (or 50, depending on where you are), but into physical problems for the machines connected to the grid. If the grid frequency suddenly drops (due to a sudden drop in generation capacity or sudden drop in load), the spinning masses connected to the grid will suddenly be under enormous mechanical stress that can destroy them.
It's obviously not possible to instantaneously increase or decrease explicit generation in response to spikes or drops in load (or alternatively, instantaneously increase or decrease load in response to spikes or drops in generation). But we don't need to: all of the spinning mass connected to the grid acts as a metaphorical (and literal) flywheel that serves as a buffer to smooth out spikes.
As the generation mix on the grid moves away from things with physical inertia (huge spinning turbines) and toward non-inertial sources (like solar), we need to use other mechanisms to ensure that the grid can smoothly absorb spikes. One way to do that is via spinning reserves (e.g. https://www.sysotechnologies.com/spinning-reserves/). Another way to do it is via sophisticated power electronics that mimic inertia (such as grid-forming inverters, which contrast with the much more common grid-following inverters).
To learn more about this topic, look up ancillary services (e.g. https://en.wikipedia.org/wiki/Ancillary_services). This Shift Key podcast episode is also a great introduction: https://podcasts.apple.com/us/podcast/spains-blackout-and-th...