Moses - you are asking the question the wrong way round.
Mixed Fleet currently get less time off between flights than both pilots and "heritage" cabin crew however the rest time must comply with legal minimums. It does.
Whether that (and the other terms and conditions you describe as "pretty crap") is sustainable in the long term remains to be seen, but commercial reality says will largely depend upon market forces. If enough people of sufficient quality are prepared to work on such contracts there will likely be little change.