Without subsidies, somebody feeding solar from their rooftop into the grid will need to sell the power on the stock exchange, usually through a power broker company that adds them to their portfolio. (In Germany this is called "Direktvermarktung")
This power broker will remotely curtail (turn off) the solar infeed when the spot prices turn negative.
I'm also don't understand why a fixed feed-in price has to be a subsidy? Is a fixed electricity price or a fixed gas price a subsidy? It could be that home owners are more comfortable selling their electricity for a fixed price and power brokers have determined that accurately measuring feed-in over time and adding remote-curtailment functionality is not worth the cost for small installations. I doubt we will see feed-in prices without any subsidies in the near future, but it could certainly happen.
And your second point is also valid, a small flat rate infeed may be easier to bill for the broker than per-minute pricing with curtailment. But this will only be the case if prices aren't negative for too long.
If it weren’t for the subsidised price and general resistance of consumers to spot pricing we would have someone offering a service offering supply management by rooftop solar installations to increase the spot price, fewer green energy certs being sold, and way fewer installs. We would also see increased battery storage.
In the very long term we would expect electricity generation to become slightly more expensive than free.
People with solar talk a big game but the vast majority of them are thinking about their pocket and just hate our energy industry.
The fact is that if anyone other than a highly niche purely spot pricing business offered a truly fair price for solar the market regulators would be overruled by the government because of the political optics.
And it’s a subsidy that is paid by those who do not have solar.
I just don’t think your comment applies at all here. Perhaps there are other markets and societies but it’s not us. And we have the most rooftop solar per capita on earth.