The two major parties in every liberal democracy that I can think of are capitalist parties. While fringe parties could theoretically win power and change the state's ideology it could only do so by becoming major parties. Otherwise they could not win power or, if they formed a coalition, they would have to share power (with capitalist parties).

The crushing of small parties is an inherent feature of liberal democracies as a matter of fact. Whether it's a corruption of the ideal of liberal democracy seems to be beside the point.

Even if the argument is accepted that small parties get crushed in liberal democracies because of corruption, the fact remains that these states are therefore corrupted (I don't think they are corrupt, but I'll ignore this semantic issue for now). The 'corrupt' rulers will never not be corrupt because they will not willingly rescind power. To not crush socialist parties is to invite socialism, which means the current ruling class must agree to having it's own power abolished. Why would it ever do that? Capitalists will never let the people vote away their power.

Even major political parties have been kept away from power for the mere suggestion of curbing (not even abolishing) capitalism: Sanders was not allowed to lead the Dems; Corbyn was not allowed to lead a majority Labour party government; Syriza was not allowed to enact it's promised reforms when it won power. It doesn't matter how many parties there are in a bourgeois state, the only acceptable option is capitalism (otherwise it wouldn't be a bourgeois state).

Liberal democracies are not meaningfully democratic. The working class(es) have no real say over policies, laws, or the economy. Whether they could do so by forming a party (it would have to become a big party before it could achieve anything, so any talk about small parties is a red herring), the fact remains that they have never been allowed to (unless you can give me an example).