Burning the plastic in dedicated facilities is the only good solution. Use it for energy, and burn less gas & oil as a result. Everything else tends to have a bigger carbon footprint.
Burning the stuff is profitable even today.
Compostable will produce methane etc., especially in your back yard.
As long as we are burning fossil fuel for low-intensity heat use anyways, (e.g. combined cycle heat/power plants), giving the fuel a temporary role as packaging before burning doesn't seem such a bad idea. Emissions treatment has come a long way. Alternative packaging solutions don't have zero footprint either (e.g. ferrying around reusable containers)
Burning the stuff is only profitable if you get it sorted without contaminants. But only a fraction of packaging is collected properly.
As long as that crap is produced, people will throw it wherever they want, and nobody will collect and sort the plastic for free just so you can burn it "profitable".
Burning the stuff is profitable even today.
Compostable will produce methane etc., especially in your back yard.