You should also consider the unique harm plastic waste in particular does to wildlife.
My point was that if you aren't littering, then the net cost to the environment isn't such a huge thing. I'm also not a fan of government subsidies in general. Your point really isn't a subsidy, it's a means of greater use, which is generally considered a good thing. Unless you'd rather have more environmental waste that isn't used for any secondary purpose.
I'd love to see more/better efforts with regards to wildlife conservation, not littering etc. I like to do road trips while on vacation. Seeing the shear amount of litter on roadways, in the mountains and everywhere but a proper trashcan that will get disposed of reasonably is upsetting. Frankly, I think that getting society as a whole to respect their environment and other people's property (littering and vandalism) would go further to improving things than not.
Now, reducing the consumption of oil/gas could reduce plastic as a byproduct and increase costs in which case the market would self-correct.