[joak]

Exactly, with renewables the marginal cost is zero (once the system is installed producing a kWh is free, no fuel to pay for).

And when marginal cost is zero price goes to zero as well (unless there is scarcity or artificial scarcity thanks to a sort of monopoly or cartel)

This easy to understand: imagine that A and B have invested in capacity production. If A sells at x dollars, B will sells at x-0.1 dollars stealing the market to A. Etc... there is no bottom since the marginal cost is zero.