Actually I believe the only "zero flexibility" players are Bruno and AWB. Maybe ETH can work on Bruno but IMO this is what he is.
The rest is quite flexible, but the problem is we want to push it too hard. Fred is not a DM/single pivot although he can "do the job there" , Eriksen is not a CM in 2 man midfield although he can "do the job there", Rashford is not a natural winger although he can "do the job there", so on and so on. I do not think the way forward is to push players out of position because we need to squeeze all attacking players on the pitch. It is quite a popular idea on the Caf though.