I hate to burst your balloon, Keon, but there are plenty of WW2 aircrew that never made it home because of FLugzeugAbwehrKanone (flak), and they were flying a lot higher than 5,000 meters. Heck, they were pushing 6,500+ and that was without pressurization or heaters. Anything over 3,000 meters for extended periods and you'll want to be sucking air from a canister.
Also, the pressure that steam operates at would dramatically decrease at altitude, even super-heated or super-pressurized steam, so you'd be seeing almost no gain from steam-powered anything at height. For those of you who live in high-altitude areas (looking at you, Colorado), think how much longer it takes you to boil water and how much quicker the water cools down. I think the anime Steamboy legitimately put all the real issues with steampunk together into one movie and solved them with a wavy-handy-ignore-the-man-behind-the-curtain type manner.
Pickle is right, you gotta land sometime and somewhere, and airships don't capture and control ground. Just like a navy can't retain control of land that it rains lead onto, it can set up some awesome blockades. (Hence, my support for the airships as navy viewpoint.)
But, wavy-handy aside, communities and nations are isolated, airships are the major mode of transportation, and land routes are limited and treacherous. That's the awesome thing about fiction, and Muse seems to have theirs fleshed out fairly well.
Edit: Fixed Pickle's name. No 's' involved.