Anything you put in a bathroom should be 20 year life easy. A 'forever' bathroom would mean 50 years!
Why not kerdi? How about Redgard: http://www.custombuildingproducts.co...s/RedGard.aspx
I have used that and like it. No reason to NOT use just regular drywall on the walls ( except the shower ). I don't know of any better product than cement board for tub and shower walls. I am definitely NOT in tune with the very latest engineering developments and products. Keep your eyes and ears open for new developments.
As far as all the water you expect on the floor, there are some basic things you can do:
> use lots of bathmats/towels on the floor
> carefully caulk all joints at the wall/floor interface: tub apron, all baseboards, base of toilet.
> install about twice as many CFM of quality, quiet, exhaust ventilation as you think you need! Consider a quite remotely located in-line fan.
Do these things and live happily everafter!