Looking good! Staining with vinegar and steel wool is clever! I'm going to try that.
I haven't had too much trouble with unprotected wood in the ground so far. In two years a handful of overhead wire poles have rotted off at the base, but most are still standing. The ones that rotted are all in areas that are shaded for most of the day, so the ground retains moisture better. The trestle bridge is made from Yellow Box tomato stakes stuck straight in the ground too. There's a plastic base on the ballasted deck which protects the structure underneath to an extent.
Not sure how Cedar compares, but I'd hazard a guess local conditions have a lot to do with how long wood lasts.