Anything that doesn’t break even suffers from its own success.

If you have a public transportation system that loses money on every rider, then more people using it means everyone has to pay more (in taxes).

This can all work out when the economy is good and taxes can be increased, but it’s an inherently fragile system. At exactly the moment when most people will be dependent on a publicly funded system — when times are tough — is exactly the same time when tax revenues drop.

By creating a system that can’t sustain itself, you are making the system more likely to collapse in a crisis.

Energy production doesn't have to be profitable, but no private investor would invest into a unprofitable business.

If energy production in Denmark would not by profitable, the Government of Denmark could nationalize the energy production, or push households to install more solar and sell the energy at predefined price to the grid, or increase taxes to pay out subsidies to make energy production profitable again for private investors. Or combine all this approaches.

For it to be market based the investment must generate positive return.

On other hand I would not find replacing all energy production with government run nuclear plants as unreasonable.

It does at least need to be feasible