It's actually arguable as to whether cotton is as bad as cotton
It generally comes down to this: cotton is fine in the right circumstances, worn by someone who understands the fabric and can deal with getting soaked if wearing it. For many situations it is not a good idea, such as when hiking in jeans in typically changeable British weather. The trousers are heavy and inflexible, soak up huge amounts of water and take ages to dry, on top of which they chill you very rapidly when wet (painful memories of a cycle ride in the rain, many years ago, which taught me a good deal about wind chill and the onset of hypothermia). That said, I frequently wear a Ventile top in wet and cold weather and it's superb, the most comfortable and best performing item of clothing I've ever owned. There's cotton, and then there's cotton.
My favourite outdoor trousers are Rohan Bags, a 50/50 polycotton blend. Even in cool and wet conditions I've had no problems or serious discomfort from wearing them, certainly nothing approaching any level of danger, but again they could be bad in certain circumstances. Polycotton dries pretty quickly, if it's not too heavy, and that's the important thing. If I'm expecting prolonged wet or very cold weather then I'll be wearing something warm under the trousers, just as I wear a warm non-cotton top under the Ventile smock. Indeed, I also have an old pair of trousers from Craghoppers where the polycotton is backed with a light fleece and they are superb for winter.
Campers and woodsmen of a century ago wore cotton or wool trousers, naturally enough. Corduroy was frowned on as holding too much water, but khaki was considered pretty good as was a high quality moleskin. Bear in mind that these people expected to camp with a fire, so they would dry their soaked trousers and stand around in woollen longjohns. As long as you take the ability to dry your clothing into account as well as the worst weather you can expect there's no reason to avoid polycotton, in my opinion. Having a pair of polypro leggings in the pack just in case can be a useful tip, however.