The main difference in a multiparty system is that people still haven’t figured out what the right way to run the economy is, and each time a different party gets elected they pull things in a different direction
If the party dictates "the right way to run the economy" as you say, then doesn't that blunt people's ability to reform the direction of their leader's policies because of the framework enforced by the party?
I'm not arguing that Western democracy provides superior remedies to public disatisfaction or that socialism is not the correct path for prosperity but, if the argument is about allowing people to meaningfully oppose the policies of their elected representatives, then, in a one party system, changing those policies also requires reforming the ideology of the party, which is an additional barrier. Multi-party systems are by no means perfect but at least they provide some alternative path where an outside party can be formed with radically different ideas that can challenge the larger parties and try to pick off support.
And, yes, there is always the threat of smaller parties being squashed using political/financial power, but that, to me, seems like more a product of corruption than an inherent aspect of a democratic system. Not to mention, the same could be done to factions within a party trying to facilitate similar reforms, no?
I think it's reasonable to argue that the almost every democratic party has this principle. Even those that argue for unfettered capitalism can see that as working in the interest of the majority and the only way the economy can be truly "publicly owned". You can argue that they are wrong but that doesn't mean they don't believe they are following those principles just as faithfully.
If the single party's ideology is so broad that it basically encompasses "don't be evil" then I'm not sure I even understand the distinction between having one party and having a "partiless" state (which would effectively make factions within the party defacto parties in and of themselves).