There is no way 2 parties can represent the diversity of opinions and ideas in the country.
2 parties means power tends to jump back and forth due to the recent ruling party doing badly vs the opposition actually providing an alternative and compelling change. This means parties tend to "lose" more than actually "win" elections.
2 dominant parties when one side of the spectrum is split among 2-3 parties tends to allow a smaller minority to achieve stronger governments which is not representative. I.E the split on the right in the 90's allowed the Liberals to have many successive majority governments despite less than 50% of support for many of those elections. In the aughts the alliance and PC merger turned that around and now the NDP and Liberals tend to split the left to a degree and the right can win a strong majority with 35-38% of the actual vote. This doesn't benefit any side long term.
"getting things done" isn't always the best metric for a political party, especially when they don't have the public support for their changes.
STV or various other methods that allow proportional results while maintaining current representation and government size were the best outcome, but didn't benefit the liberals so they dropped it.
Countries with multiple small parties frequently seem to collapse into political torpor where nothing ever changes.