If an Airbus pilot can’t do a sim in a 744 or 737 then they’re not a very good pilot and I’d question their capability as an Airbus pilot too. The same applies the other way too. It’s about rate of learning. It’s about trainability. It’s about core skills.
However let me put it another way, if it’s not needed then why do people still fail it as not meeting a minimum standard? I’ll answer that for you. They fail it for the same reason that lots of doctors don’t actually become surgeons, lots of law graduates don’t actually become lawyers or barristers or judges.
Are you seriously suggesting that Air Ambulance don’t do a flying assessment of their employees before confirming their employment?
This was brought up in a different thread, and it was pointed out that while an Airbus pilot might struggle, how is that any different from someone flying a SAAB or a Titan? As Keg has pointed out, the assessment standard to reach is the same for everyone.