Quite simply, if solar power becomes cheap to buy and put on a house, it also starts becoming cheap to buy and put in a power plant not too far from the house. If you could generate solar for 5 cents/kwh, why would you burn a fossil fuel that ended up costing more? If grid power is 40 cents/kwh and solar is 10, then people will just put up solar plants on the grid.

Now there are ways it's not quite that simple. Grid power suffers from grid losses and the cost of the grid. The closer you can put your plant to the customer, the better. Home solar takes advantage of a "free roof" and "free land" and power plant solar doesn't do that as easily -- though it might well be able to lease existing land being used for parking lots, or otherwise wasted roofs.

Solar only provides power when sunny. You have to have a grid for cloudy days and for night, and that grid has to come from power that works reliably then, such as hydro and fossil fuels. However, there might be enough of them.

But in the end, grid power can't get vastly more expensive than solar. And new techs will appear on the grid as well.