Before the x23B, we had a 14-year-old pop-up camper and never had any canvas leaks. We've had the x23b out in a heavy rain storm, and were quite dry. As you're used to tent camping, the only thing you'll notice is occasional condensation on the canvas on some cool nights. With normal care (opening it to dry if packed wet, checking the seams and attachment points periodically), I don't see why the canvas wouldn't last.
As far as when the kids get bigger, if we're still using this camper 5-6 years down the road, the plan is to have the 2 girls on 1 bunk, and then open up the couch for the 2 boys, or do a combination of the couch and a light cot (which we already have) for the 2 boys. Then we'd still use the bunk by the dinette. We don't envision folding the dinette down to use as a bed unless the grandparents come with us.