The dominant reaction to the discovery of synthetics such as bakelite and PET seems to be excitement about their properties and their potential use. The environmental impact seems to have not been considered at all for a very long time, as far I can tell. I can't exactly exclude the possibility of some minor party raising concerns about the non-degradable properties of the new materials, but if one has, then it has fell on deaf ears, and in general it seems unlikely given, that your typical chemist/scientist did not concern himself with the subject of environmental impact.
To better illustrate this, I'd like to point to L. Baekeland's notebook. Baekeland was a chemist who had already done several discoveries, and in 1907 serendipitously discovered Bakelite ^(polyoxybenzylmethylenglycolanhydride) -an early synthetic resin that quickly became very popular in manufacturing. In his personal notebook, he not only writes about his general work and the discovery, but also expresses his personal views on many ethical and philosophical issues such as workplace safety, consumerism and animal welfare. The man was somewhat of a character in this regard, who always eager to discuss all kinds of concepts and concerns with other intellectuals. This is a man who, in early 20th century did experiments to try and turn saw dust into edible fodder for livestock to lower the food cost of feeding animals. Sounds like the perfect man to also consider the potential negative impact of mass-production of a completely "unnatural", chemically non-reactive, non-biodegradable substance, right?
Except he never even considered it. He was far too occupied with improving and spreading his new product, and this part of history flows over from all the scientists and engineers wondering about new applications for his synthetic material. The first considerations for where these plastics eventually end up and whether we might be dumping too much of it in our 'environment' is only in the 50's, loooong after many more successful plastics have entered mass-production, with some of the first academic studies on plastic consumption by animals. Not coincidentally, this is also the period where many modern ideas about our environment are formed and awareness starts being raised. I cannot stress enough how such a mindset as is found nowadays, was not present in the same way 100 years ago. The chemists and engineers working with new plastics were invariably enthusiastic about the new properties of the materials that they got to work with. PET was a war-time secret because of its important radar transparent properties. This is radically different from how new materials and techniques are viewed now (case in point: carbon nanotubes aren't even out of the lab yet, but environmental and health concerns are already raised).
Leo Baekeland Diary Volume 01, 1907-1908