Aren't all of those things only a tiny fraction of our oil usage though? The entire global plastic industry accounts for < 5% of the oil we dig up IIRC. The vast majority is turned in to fuel.
The bottom line is these companies are being valued by the market based on the assumption that they'll be able to sell the barrels of oil in their proven reserves. If those reserves are "unburnable", the current valuations are not accurate, because the market for their product will be much smaller. Either they are valued correctly, or we're going to take action that prevents the worst effects of climate change - but it can't be both.