Canvas from hemp?
My guess: The early European sails made from canvas should have been made from flax or maybe wool, as I do not think hemp was grown in Europe.
Maybe hemp came after that, when trade with Asia developed?
Yes, modern Cotton growing is not so fantastic for nature, but when it decomposes it is fine. I suspect though that the natural fibers ( hemp, flax) are also grown using pesticides, fertilisers and so on. Unavoidable in an efficient agriculture.
Not sure how Organic growing of these is done.
Synthetics give a long term negative effect. Natural fibers do not.
I do not know myself how Viscose is in environmental impact. Maybe somebody else here can teach us! I know though that Viscose (bamboo fiber as start) is marketed cleverly as 'bamboo fiber' or even 'bamboo silk'.
Synthetics in some uses are not as long lasting as cotton. Nets and lines for marine use - properly treated cotton outlasts synthetics.
But it is high maintenance.
I have an old handline, made before synthetics became the norm, I use sometimes for fun. Given to me by my old friend, 91 years young.
It was made just after WW2. Even the hooks are that old.
After using, it needs to be spooled up on a special frame, desalinated in fresh water, dried, hooks oiled in. This process takes a couple of days, it dries slowly.
After several used and cleans, it needs to be preserved using some kind of tree tar oil. ( not done that yet myself).
It is the only handline I use, as it is nicer on my soft city hands than a nylon line. Maybe 4 times a year. But he used it about one day a week for more than 50 years. He had/had maybe 10 of these lines, as they have to be rotated in use.
Buying a synthetic item of excellent quality and using it for decades - nothing wrong with that. But people tend to buy cheap stuff that does not last. Buy, use, throw away. Landfill in best case.