I’ll agree that the wealthy have more control and influence than the average person. That said, multiple parties at least makes it more expensive and difficult to influence politicians. As it is now, you only have to contribute to 1 of 2 parties in order to push your agenda. If there were 3 or 6, you’d need to push several in order to gain the same amount of control.
We have over a century of evidence that it’s not difficult at all to influence politicians, especially when there’s practically no working class representation in politics. Also, when you have a bunch of parties then they all just pull things in different directions making it nearly impossible to do any sort of long term planning. This is why a lot of large infrastructure projects China is able to do are basically impossible in the west.