> This is to incentivise the cheapest production methods to be invested in.

It's also just a rule of economics. The price is set at the cost of the most expensive production necessary to meet demand.

So if solar could fulfill 100% of energy demand, price would be the cost of solar, and any other more expensive generation would either lose money, shut down or idle.

But if we shut down or idle those today we wouldn't have enough electricity, so the price rises until the more expensive plants can stay open and demand is met.