The people I know who continue to support things and people that have long since been proven to be a crock have two thing in common.
Firstly, they think the world is out to get them.
So when you say Brexit/Johnson/Trump/Whatever is proven disaster they don't think, hmmm, maybe this person is right - I'll think this through. They think, 'why is this person saying this about the one reliable point in my life?' 'Why are they trying to undermine the thing in which I have invested all my faith in this dangerous world full of people out to get me?' 'What's their motive?'
Any reconsideration exposes them to a dangerous world of new people, ideas and concepts and that's hard work.
Secondly, to protect themself in this dangerous world (of conspiracy theories) they had taught themselves to always be the one in the right, the one who has seen through the lies.
To back down on any argument brings that safe wall crumbling down. They know best. Any data or evidence is dismissed as it comes from a lower sourse i.e. not them.