I think the most fundamental change of Brexit is the overwhelming "you're not welcome" message. Visas, papers, bureaucrats, people can deal with. But the idea that you're at best tolerated so long as you pay your taxes is really off-putting (even for me, now UK citizen and long-time resident).
Also, all these schemes forget that moving for a job is so much more than that - it is moving a life. Can you move your life with your partner and kids - are they welcome? Can your parents come and visit? Is it obvious you can access healthcare, schools, social safety net on fair terms? Will you have to justify your whole existence to a reluctant bureaucrat every 12 months?
Pre-Brexit, there was always a bit of tension to foreigners in the UK, but then there always is, and London was by far better than most places. But the guarantees of EU membership were so strong that you didn't even think about any of these issues.
My impression is that another upside Silicon Valley has over the UK is that the US is genuinely a country of immigrants. Whatever hoops you have to jump through to get in or stay, I don't get the impression people are looked down on because they are foreign, once actually on the ground. Which is not so much the case in Brexitania.