While a specific party within bourgeois parliamentary democracy can be said to be a progressive historical force and enact quantitative changes, as long as they are forced into coalition with bourgeois parties then they are forced into a qualitative position of class collaboration. That's why parliament's composition matters, it's still a dictatorship of the bourgeoisie.

A revolutionary agenda determines whether they can turn a majority into qualitative change, but no matter how revolutionary their goals might be they still aren't actually in command at the moment - though that isn't to say that they need a majority to achieve a revolutionary agenda! They could seize power in some other way. What matters is who is actually in control, and at this point it isn't the socialist parties in control of the government or economy.