As I have said on various occasions, my wife and I would prefer paddling a leaky kayak to Alaska rather than go on an NCL ship. Let's just say, the mattresses you describe would not surprise me...but that's just a useless personal opinion.
$5000 for a cruise to Alaska plus pre- and post-cruise hotels? Seems reasonable depending on how budget conscious you are.
HOWEVER...keep in mind that the real value of an Alaskan cruise is "Off the Ship." There is so much history, culture, and environment to experience that it would be a shame to miss it. The state of Alaska puts out a great guide with tips on where to go, what to see, low-cost excursions, etc. You'll save all sorts of money booking your own side trips compared with what the ship offers. Also check out we sites for the individual towns you'll visit.
Alaska is a phenomenal experience. Go make the most of it.