Two party system is just a bad idea if you want to have options. It’s basically left or right and if you’re not pleased by their politics, what can you do, not vote or vote the other side even further from you?
then there is only one coalition
I don’t get what you mean. There’s a huge variety of possible coalitions. All depends on who can make the 50%, if they’re going for a majority government.
So, the question is only if you are the one deciding what compromise to make, or party you vote for decides for you. There is argument to be made that it is better for democracy that you decide for that.
You are absolutely not the one making those decisions with two party system with two big tent parties. They’re making those decisions and compromises inside the party, since they have to please most of the voter base anyway and try to fish for new voters from the other side.
Each party in 2 party system tries to maximize number of votes and adjusts its position for that as well, which is similar to “power balances shift inside those coalitions”
The established party leadership in the US seem to just do similar sort of politics again and again, and reacting fairly little to that sort of stuff. It’s nowhere near the difference from voters switching to a different party altogether.