When the plastic had not yet been discovered, metal or crystal alike materials were much more used. Although like always with some exceptions, now glasses seem reserved for products with certain price and sense of quality, like wine, mineral water, sauces, virgin oil, vinegar and so on.
In a glass factory, there is some combustible involved to achieve high temperatures to melt the raw material and create the desired shape. Sulfur dioxide, nitrogen oxides, fluoride, sulfates, alkali, chloride to mention a few, but of course this product it's not mandatory if electricity coming from renewable energies is taken into account. The melting point of glass could be around 1500ºC, while plastics could experience a much lower one of 250ºC.
The price and convenience for producers look the primary factor in the shift toward plastics of a single use. Also to consumers, but all from a simplistic point of view and then avoiding the complexity of analyzing what could happen for this cause. Plastics should last more than a century and even some touch a millennium, and a high quantity of plastics aren't recycled. There are studies suggesting that different plastics should not be in contact with food because they expel harmful particles in specific circumstances.
After an incorrect disposal, little PET chains are capable of passing to the food chain if they were previously degraded. They are toxic in general.
The good esthetic properties of a plastic normally decay with the use, as scratches or other imperfections appear to being permanent due to their inherent characteristics. On the other side, crystal is more resistant and could be reused for thousand years if there is a sophisticated process to renew and disinfect the material.
Anyway, a polymer based structure is not obviously the problem because with a more evolved recycled industry and increased overall care, it's possible to create a closed circle in which plastics don't create environmental impact of any kind.
The critical part appears with the current disastrous use and intrinsic wasteful objectives where the quantity of waste per individual person exposes too many people as guilty of entering in a behavior which condemns the health of the environment and with it, their own.