Any fabric, especially the lightweight stuff we like, has a limited life expectancy. And of course the more it's abused, the shorter it'll last.
As far as your tarp goes, it doesn't sound like there's anything wrong with it. Silnylon is only waterproof to a certain psi - many have reported heavy rains getting through in the form of the "mist" you're talking about.
"Physics is the only true science. All else is stamp collecting." - J. J. Thompson