When you have one big producer (one big hydro-electric dam or coal power plant), then stabilizing the frequency is trivial, because you only have to talk to yourself.

Your frequency is still influenced by a million and more consumers. And that’s before blind currents come into play.

When you have 100000 small producers (assume everyone in a bigger area has photovoltaics on their roof), then suddenly stabilizing the frequency becomes more challenging, because everybody has to work in exactly the same rhythm.

…and? Everyone is getting the current frequency via the network, everyone knows what it should be, everyone can do their own small part is speeding it up or slowing it down.

The actual issue is that huge big turbines have lots of inertia which, through their inertia, naturally stabilise the frequency. On the flip side inverters (like with solar panels) can regulate the frequency actively, what’s iffy is smaller AC generators like wind mills. But then there’s also battery and capacitor banks.

It’s a thing network engineers have to worry about, but it’s not some insurmountable problem. We’re already doing it. Insular networks have been doing it for ages, e.g. in Germany Berlin’s network wasn’t part of the eastern one and they always used stuff like capacitor banks to stabilise it.

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All that aside yes in the future there’s probably going to be a high voltage DC network in Europe. Less so for private consumers, at least not in the foreseeable future, but to connect up large DC consumers, that is, industry, with DC power sources. If you’re smelting aluminium with solar power going via AC is just pure conversion loss.