> I don't get the argument since when it's no longer economic it will presumably be shut down.

You should read the (excellently named) No Country for Coal Gen report, which argued in detail that closing coal power plants was the economicly efficient thing to do but that various regulations meant they'd stay open long after they became net negatives costing tens of billions.

https://carbontracker.org/reports/no-country-for-coal-gen-be...