Look closely... they are "commercially compostable" which means heating them to something like 200 degrees for multiple hours. Throw them out in your compost pile or a landfill, and they'll still be there forever, or at least a really long time.
I suspect balancing "speed to degrade" with "stability when used" is something that will consistently improve with technology. (Not to mention manufacturing.)
The problem with trash isn't the space it takes up while it breaks down. It's that you're taking all the leftovers from petrochemical processing, making them into disposable products, and them dumping them en-mass into the "biosphere". (I consider a dump effectively the "biosphere" when compared to "a mile under rocks sequestered for millions of years after growing from CO2 with solar power".)
Not only does this mean there's more carbon on the surface, but there's all the gross stuff that came up with the oil that has a tendency of not breaking down in the environment, bio-accumulating, etc. (It didn't break down in the first million years... But stuff that never breaks down is such great packaging!!!)
