People were saying 'How are the prices going to go up even more?' years ago but they have gone up; that's why a 3-bed flat in Kentish Town is now £500k.
I moved out of London but still in the SE so I would be considered a mug too. Are people who live in Manchester mugs because they choose to live in Manchester rather than in an ex-mining village further north where you can get a 3-bed house even cheaper, say for £60k? No, because they're choosing to live where there are plenty of jobs on offer plus other amenities and opportunities that fit with their lifestyle.
I say this as someone who has lived in a village in the NE, a city in the NE, London, and now a village in the SE. No one is a mug. Where people live is influenced by where they need to be for work, where they want to be to access culture, countryside, education, sports and arts facilities etc., and where their support network/family are based.