In Medical fields, 457 holders get paid the same contract and payments as Australians. they cover essential parts/areas that Australians can't and don't want to cover.
They are subjected to stringent international standard testing and scrutiny that they even get downgraded initially from their original position (just in case). There are still much needed vacancy in specific medical and nursing specialities in certain areas used to be called (Area of Need).
In other fields, it should have been illegal to be getting paid less than permanent residents or Australians, constitutional point of view.
Also, it is true 457 visa is cancelled but those professionals will get entry under different visa name. There are still in Occupational shortage lists. It might not be as easy to get Citizenship after, which might reduce the incentive to those skilled professional to come over.
(I'm not sure about citizenship pathway to the new visa holders now).