Double up the bottom!
I have been making bags for almost 10 years, so I've seen a lot of 10 year bags. To get a 10 years + bag I opt for 1000D Cordura nylon. Bigger cost at the start (and it's synthetic), but the bag will last for decades unless you are an abusive bike messenger. Canvas facing wear + UV (+ salt air for you) might wear out faster than you think. Waxing it will help.