The reason that £30,000 has been chosen is because research from the Migration Advisory Panel shows that (on average) it is only migrants earning over this threshold who actually contribute more to society than they take out.
We also have plenty of nurses and midwives. They are just not working very efficiently.
And what Windrush 'scandal'? A small group of people post 1973 did not get round to regularising their immigration position in spite of extensive information campaigns by the Home Office over the past 40 odd years?
It's about as useless an excuse as having an unlicensed TV after TV licenses become obligatory or an uninsured car after car insurance becomes obligatory. It's a waste of time saying 'but I was legal in the past'. Tough.