Of course it's more complicated than that....

Modern waterproof-breathable fabrics include several crucial features, any of which can be done at any level of sophistication and function. The one everyone pays attention to is usually some sort of membrane like Gore-Tex or eVent. You can't renew this. In my experience the membrane usually splits or degrades in some other manner with time and use, allowing liquid water to penetrate. When it's broken, it's broken for good. Quality construction will slow this degradation, but it'll happen with even the most expensive gear.

Another factor is the surface treatment, generically named "durable water repellant," i.e., DWR. When you buy a jacket brand new, water beads up and slides off it without wetting the fabric fibers themselves. That's the DWR coating at work. You can renew this coating using stuff like Scotchguard or (far better) wash-in products like Nikwax. This will really help, although your gear will never be as good as it was when you purchased it. All manufacturers include instructions for renewing DWR treatments on their products.

Then there are weatherproofing construction features. I've worn lots of gear which leaked due to basic construction defects--zippers located right where water pools in use, vents which aren't adequately protected against drips or blowing rain, that sort of thing. One way Gore promotes its (somewhat misleading) "Guaranteed to keep you dry" slogan is by tightly controlling the physical construction features in garments authorized to advertise their Gore-Tex membranes. Other companies have their own proprietary membranes, therefore their own design standards. Read up on Patagonia brand design features some day to get a sense of how a high-end manufacturer deals with waterproofing issues.

I'll add only that I find motorcycling gear sadly deficient in this realm--lots of very basic design defects compared to boating or skiing clothing. For example, no boating jacket would have pockets which admit water without drain holes to let the water out. Not only will un-drained pockets fill with water which will eventually find its way into the interior of, say, a jacket, but anything you store in that pocket will float around in its own little lake until you notice that your registration and insurance documents have turned to illegible mush.

Waxed cotton is a very different approach, and it's been so long since I went this route (heavy, stiff, waxed cotton tents and tarpaulins were state of the art before the advent of urethane coatings, then breathable membranes like gore-tex) that I can't really address the issue.

Caveat: the above represents a hasty assemblage of half-truths and vague hints at the complexity of the subject. People spend their entire professional careers working on this stuff--either providing better products or obscuring basic inadequacies of the ones they're hoping to sell you. All I'm really offering here is a caution against settling for simplistic answers...and hopefully offering a framework for looking into the issue further.

Mark