The ATPL is the almost the highest qualification a pilot can hold. The maritime equivalent is a Master Class One. Responsible authorities do not hand these out on a plate, for good reason.
By not having an ATPL test, or having a really dumbed down test, we would be descending to the level of places like Liberia, where a bag of cash and my sailing logbook would probably get me a ticket to command a super tanker - Liberian registered of course.
The trick with regulation is to strike a sensible balance that will meet ICAO standards - but not exceed them by an unnecessary margin - while recognising that most countries will have some variations to meet local requirements, whether political (industry pressure often to reduce requirements versus public expectation towards high requirements) or as a result of a unique climate or environment. Australia is not all that unique, but we are very politically-driven and of course WE are THE super nanny state.
So, an ATPL test in a Seneca, I would not buy that, but a baby King Air or Citation is just about complex enough despite not really being multi crew.
Should the test be multi crew? Absolutely. If you want to fly single pilot what the hell do you need an ATPL for anyway? Oh right, to get a fast track command on a big jet.
...ducks for cover......