Why shouldn’t we call out Lib Dem “betrayal”? Because they haven’t betrayed anyone. To think that they have reinforces the mistaken belief that, when they describe themselves as “progressive,” they mean “left.” But Lib Dem progressivism isn’t just a fluffy sort of not quite socialism, it’s a specifically liberal version of progressivism.
Consider, for example, welfare provision. The issue here is not simply one of more or less state support, but about how that support is provided. Conservatives don’t actually want (too many) people starving in the street; but they do want those who receive state support to be directly disciplined, probably by highly moralizing institutions (hence the conservative support for certain kinds of religious charity). Liberal welfare provision, on the other hand, requires that the recipients be disciplined by the amorphous institutions of the market.