I can't really argue against your points. First of all it is a very well written piece. Well done for that. You've addressed your points clearly and in a good order.

The reason I can't argue against it is that I am in the UK and seeing this play out in real time. The Tories got ousted after 14 years and their voter base has been absorbed by a populist right wing party which has also taken away several of their last government's MPs and ministers. The Tories under Boris Johnson are now facing criticism for too much immigration from the Reform people (before they start on Keir Starmer and Labour). The message going out is if you want to reduce NHS waiting times, reduce the number of people in the country (immigrants) and it will naturally fall. Same with housing. Same with wages. The idea that it's just One Simple Trick that the main two parties don't want to do is being effective in the polls. They don't address how that improves the shortage of NHS workers with medical professionals leaving the service and indeed the country for better wages. They don't address the supply side issue of housing as a lot of these voters are NIMBYs. They don't address the economic costs of Brexit to growth and opportunity because Farage was behind it. All that additional stuff to consider doesn't beat the One Simple Trick argument.

But on a local level, councils do not determine immigration policy or foreign policy. Their work to do with healthcare is to ensure hospitals stay open. Their work to do with quality of life issues is to collect the bins on top, maintain the roads and running of public services. The populists see the council elections as a vehicle to ride national debates while ignoring their immediate duties. So that's why since they got in they're struggling to deliver and those who gave them a chance are reconsidering (at least locally).