It is just that the two party system is a concept of 2 major parties that conquers other parties in number and power. To have multiple major parties, parties in politics should have almost same number of members to balance the numbers. BUT, is there people willing to do this? and if there is, how many? will others risk going to minor parties instead of going to the major ones?
I don't think the number of parties is the issue, so much as the electoral system itself.
A first-past-the-post, or winner-takes-all system will always be unrepresentative and will always favour big parties, and cause people to not vote for smaller parties because they will be throwing their vote away.
Take an election where the result is:
40% red party
35% blue party
15% yellow party
10% purple party.
First-past-the-post means that red and blue will take almost all of the seats, and a vote for yellow or purple is pointless.
But proper proportional representation would give the seats in accordance with number of votes, 40% of seats to red, 10% to purple, etc... and would be so much fairer.
What you say is true for a head of state, where there is one winning for the entire nation/state, however this is not the case for representative elections, where one representative represents a district. If there are three or four parties in the legislature, the parties will be forced to compromise to pass a budget and/or legislation.
To further improve a multi-party system, the number of districts could have halved, and each district would get two representatives, the candidates who receive the most and second most votes, and each voter could cast two votes in an election (but could not vote for one candidate twice).