The opening response is always the same - "natural materials are too expensive" or "have not been technologically developed yet to be light and comfortable."  We are stuck in the mindset of technological progress and expect sustainable solutions to just appear in front of us.  Yet the only way to allow this solution to become available in shops is to actually stop buying the alternative, synthetics, which we don't want.  You don't need a Gortex jacket and if you really want one that's good for traversing glaciers - it's going to cost the same as a cotton gabardine raincoat which is essentially just as good.  Yes, the initial investment will be high, but the more we collectively invest in gabardine rainwear the cheaper it will become, it's basic supply vs demand economics.  Retailers are fully capable of stocking gabardine from makers such as Mackintosh Ireland, or Grenfell who make the best and most affordable cotton rainwear in England.  If you demand it, they will stock it, if you complain but still accept it as the best solution for now, then Makintosh Ireland for instance or a company based cheaply in Bangladesh for instance will fill the price gap with a cheaper selection of cotton rainwear, which would be the best solution for us all right now.

Rainwear is a technical garment, it's something that takes considerable effort to produce and so naturally can't be very cheap - if it is sold cheap then someone's paying dearly for it, with respect to synthetics, it's all of us and life as we know it.  

The history of waterproof fabrics begins with wool.  Before gabardine was invented in 1879, there were the following options, oilskins made of cotton covered in an oily substance, heavily felted wool and boiled wool.  Oilskins are most synonymous with Barbour, felted wool with Tyrolean tweed or loden and boiled wool with duffle coats.