They could all have a qualified pilot on board for emergencies - it's just that most of the time they'd be occupied with serving the snacks.
My assumption is sims will still be available...
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"?
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)?
Frankly whilst I'd "never say never" I still don't see the cost vs. benefits adding up in favour of the pilotless or emergency only pilot concept any time soon.