That “assuming nothing else changes” is doing a huge amount of heavy lifting.
Something that I heard a few years ago really demonstrated people’s rightward political shift as they age like nothing else.
Imagine a child born shortly after the end of the Second World War, say 1948-1950. That child would have been a young adult, 18-20, in 1968. That was both the year that the hippie movement gained greatest prominence as well as the year of radical protest where young people around the world organised and fought back against corruption and repression.
Now fast forwards to 2016. Those very same post-war children are now aged 66-68. That’s the demographic that more than any other voted in favour of Brexit. I bet if you’d gone back to those young radicals of '68 and told them they were going to become bigoted, narrow-minded xenophobes they’d have laughed in your face. But it happened.
But that’s the whole problem. People haven’t been turning Tory as they get older. So the Tory base is aging out of the population.
The Torys probably have at least another term of crazy, before they get their ship in order. They’ll have some new “hug a hoodie” centrist leader and stop being so visibly the naughty party.