Simply getting it back to pre-Blair levels, and amending the Blair-era laws that prevent us deporting foreign criminals, would go down as a massive immigration win for the vast majority of voters, and, as you say, probably lead to them winning elections however and whenever they like. They just don't want to do that. You can see this across lots of areas. If you can't build in nice Conservative-voting areas, why not stuff cities full of development and potentially get some more young people voting for you? Why not build new prisons and reservoirs in Liberal Democrat seats? Other than a brief period in 2019, which secured them a massive majority, they've spent fourteen years chasing people who will never vote for them under any circumstances.