As another commenter pointed out, you could (and people surely did) make the same argument about lead or asbestos. That alone doesn't mean it's not the right thing to do.
Why is it impractical? It would be unpopular and put prices up but it could be done. We would have to reduce the amount of clothing that is produced but that is hardly a problem as we produce vastly more than we need. We could eliminate fast fashion for a start.
You don't hear people saying, man, that asbestos and lead did a bang up job on X. But you 100% see 100% of women wearing stretchy pants. For the longest time I exclusively wore cotton jeans and then they became impossible to find and I finally realized what plastic was good for: fat people. Now I see men wearing them all the time. The struggle of the 80s on people killing themselves to fit into jeans is not longer a thing.
Not sure how socks used to be without plastic but pretty sure they'd fall apart fast.
You don't need plastics for that.
Warp knitting has been used for the last few centuries with cotton/linen/silk/wool to get stretchy textiles, and there are plenty of stretchy pants that are made of natural materials available today for those who struggle fitting into jeans.
In my opinion, no natural fabric is as comfortable as synthetics. But I have cotton bedding and curtains. I think these are higher priority: bedding is used while you're lying close to it and easily able to inhale any dust, and curtains are exposed to sunlight that can weaken the fabric and make it break down more easily. They're also both large, meaning there's more plastic to be released. Carpet also seems a high priority, although that's more expensive and difficult to change.
I do hate fast fashion, but I can think of many exceptions where natural fibers won't cut it. There is no natural fabric product on the market that can properly replace lightweight, waterproof clothing. Waxed cotton is waterproof but heavy. Rubber is not breathable. Technical synthetics like Goretex have its issues (low durability, high cost, coated with PFAS), but it sure beats getting hypothermia.
It's a rare exception that natural fibers won't serve. Alaska, Arctic circle, Antarctica, disaster prep.
You don't need lightweight waterproof clothing, you just like it. Getting a little sweaty under a non-breathable fabric or getting wet but staying insulated with wool will be just fine. Umbrellas are great as well.
What beats lightweight breathable fabric is not desecrating the planet.
People will make all kinds of excuses as to not impose any minor inconvenience to themselves, even if it means making species go extinct and killing the planet.
We can replace Nylon with get this, poorer alternatives, if we gave a shit about the environment. I know it's unthinkable to switch to a worse product or user experience, but imagine caring more about the health of the planet.
This feels naive. For things like parachutes or airbags, I question whether you would actually have benefits from using something like cotton. In particular, they almost certainly do not perform at the same level. As such, you run the real risk of losing gains from other material to losses to maintenance.
Some people would just prefer that more people die from lack of effective parachutes or worse airbags if it means reducing the environmental devastation we wreak on the planet. Not sure if that is something you've considered.