Let's put it this way. I don't have evidence of 16 spoke rear wheels breaking spokes with heavy offroad touring either. But for you it likely will not matter. Why wouldn't 16 spoke rear wheels work if a manufacturer were to put them on a bike

I only really have my experience and knowledge of reading up on the subject and doing my own bike mechanics and building my own wheels and others for the last 10 or so years. And I'm not saying 28 spoke wheels can't work if they're well enough specced. I'm saying the trek 920 wheels aren't well specced. And I do have experience with bontrager rims. Crappiest rims I ever built, man those were horrid.