The bit you're missing is how hard it is to get yourself established in London in the first place. It's not free. As a northerner who moved down south, I can tell you that we were substantially poorer than all my northern relatives in every way for years.
We lived in shite rented places long after all my northern friends had bought property, and then lived in flats while similarly paid family up north lived in mansions.
The Londoners who are moving there will likely have had a similar crap standard of living for years in order to afford the mortgage on their tiny house in London.
All that's happening now is that they are breathing a sigh of relief that mortgaging your right arm actually buys them something worth paying for.
My husband and I have been killing ourselves for years working full time in stressful, senior jobs to afford the huge mortgage a super modest house in London suburbs.
Our mortgage would terrify all my Northern family who all owe substantially less than £100k in mortgage on homes worth at least three times that.
If we move elsewhere, it's not like that money was free. It wasn't, we paid it off over years. It's not a freebie, but expectations of property are different down here.
Almost all the professionals we know our are or younger are living in flats with kids, and they make the best of it. If they were lucky enough to get a job out of London, who could blame them for moving out?
The housing market is a tragedy for anyone younger, but has made paper millions for people who had reserves of capital at the outset. Its a disease that is spreading outwards from London, but it's caused by policy, not people.