Update: I made a silly math mistake. That's $9 profit per shirt. So if you make 100 shirts but only sell 50 and burn the rest, that's $450 profit. You make $4.50 per shirt manufactured.

Stated another way: you can total up the manufacturing cost of the shirts you destroyed ($50) and distributed evenly among the ones you sold (50/50=$1 each) and just add that to the cost of each shirt you sell when calculating profit. Same result.

This would been that more competition would be good for the environment because it would drive down prices and margins, and thus the incentive to overproduce. But this rule actually decreases the competitive pressure and increases margins because market exit barriers = market entry barriers

If you throw some plastics into a coal fired power plant it is almost the same as if you would burn oil.

There are anecdotes about trash incinerators requiring less kerosene to finish the burn, because of the plastic content.

I have a feeling nobody is paying particularly close attention to capturing usable energy from this process

There is in France a kind of shared network of hot water used to heat up our homes (well, those that are connected and paying into the system at least). Part of the system works by burning trash and capturing the heat in the process. Supposedly they also work on using renewable energies to do the work.

Some people argue that the whole system is going against the objectives of recycling stuff but at least it's better than just burning it to get rid of it.