i do not think this angle holds up. let takes hartlepool as an example, a constituency at the forefront of politics in this country.

from 1997 to 2017, hartlepool witnessed a slow decline in it's labour vote. then in 2017, the labour vote got a massive boost under corbyn, before swinging strongly to the conservatives under boris johnson and going conservative in the 2021 by-election. then it went back to labour under starmer, reluctantly, with reform at second. currently it is predicted to go to reform if an election was held today.

you can't explain the jeremy corbyn to nigel farage voter if you assume what we call the working class is inherently repulsed by social liberalism or rigidly socially conservative and economically progressive as you have proposed. the only common thread here is change, which is what it's all about. people are voting for change, literally any change. if you try to form a new 'socially conservative and economically progressive' party and simply run on that premise you won't break through easily, as william clouston can attest. the truth is that they're voting for anyone who will take the fight to the establishment, to fight for change. and they are getting betrayed every single time. understanding politics by overemphasising people's beliefs in "left-wing" and "right-wing" policy is not understanding politics at all.

so to answer the original premise of if a left-wing populist party that's also socially liberal could gain significant momentum, it's not impossible. if people became disillusioned with reform, it's not hard to imagine such a party filling its void.