I'm sure they would be....
At the moment we do 2 X recurrent sims every six months, very much practicing for, as Huck rightly says, "the bad days". The rest of the time the flying we do on the routine sectors helps maintains the skill base we might have to call on for the bad day when we have to don the cape
If you are suggesting your proposed combined cabin crew member/safety pilot only retains competency in the simulator then how many times a month do you think our potential hero/heroine would need to be "in the box" (and away from the drinks trolley!) in order for them to maintain the required standard of flying they might need for the "just in case"?
On reflection, I'm sure you're right :-). The pilot-steward idea probably isn't practical - the pilot on the ground option is much more feasible. The idea of having someone on board trained for ground operations might fly (or rather not, of course) if you'll forgive the pun, but otherwise no.
You do make more excellent arguments for why the ground-based crew should be ATC/airspace based rather than airline based. An airline's ground pilot may never actually have to do anything in the real world, provided these mooted automatics that BEA are testing are up to the job. Airlines maintaining their own fleet of pilots to sit twiddling their thumbs is going to be expensive and wasteful, much more cost effective to centralise them.
Centralising them of course will also neatly sidestep the existing pilots union agreements with the airlines of course - new employer, new role, new contracts.
Have you any idea how much a modern simulator costs (because more simulator sessions means more simulators), and do you know how much they cost to run (I'm sure someone here knows)?
No idea, other than 'incredibly expensive'. Since variously over time this place is full of people moaning about how basic flying skills are what is lacking in modern pilots, I've often wondered if throwing 'em up in a light aircraft once in a while might not be cheaper than doing everything in the sim. Type training needs to be in the sim of course, but a Cessna would seem equally effective at teaching basic airmanship. That's another discussion of course.