I'd be 100% ok if all containers were mason jars. Most of the time, I can't even reuse glass jars because of their stupid, non-standard lids!
Cereal should just come in a biodegradable plastic, no box.
But standardize everything. Never mind hassling consumers not to use plastic bags when companies are putting layer upon layer of plastic on their products. If a manufacturer can't use a standardized package, they shouldn't be allowed to sell the product without a massive environmental tax added onto their product.