There's infinite potential jobs. Doing anything useful is a job.
That said, the Australian and Danish systems are the best because they're more flexible.
[0] This holds up to about 60% of median wages. You can imagine it'd lower the hours some people get before it entirely makes them unemployed.
[1] One is that it provides price signals to monopsony employers. Another is that it reduces search costs in the labor market by basically acting as a spam filter that gets rid of time-wasting job offers.
That's because of agglomeration effects I think. "In the UK" is the keyword, if they wanted to earn ten times that they could move to Silicon Valley and do that. So you're left with people who don't want to uproot their lives like that.
But wave stickiness is sorta just part of human nature.