You are right that such changes are rare, they certainly don't happen once every 25 years, but the fact that they can happen is really important. Otherwise you are left with situations like current USA where many wants to vote for lower taxes but don't want to support Trumps other political views, in Europe you just vote for another right wing party but what do you do in USA?
Similarly you have many people who wants to vote for higher taxes and more government programs, but don't want to vote for all the identity politics. What should they vote for? Now the entire left is associated with identity politics and the entire right is associated with opposition to identity politics, making it hard to distinguish between different views and probably making the whole political conversation way more toxic than it needs to be. It has gotten to the point that people often assume you are racist if you argue for lower taxes etc.
MMP in NZ has increased competition and brought effective collaborative governments, and it hasn't even been 30 years.
(For reference, the Canadian House of Commons, according to Wikipedia's footer, divides into 158 Liberals, 119 Conservatives, 33 Québécois, 25 New Democrats, 2 Greens, and 1 Independent.
The Senate divides into 41 Independents, 20 Conservatives, 12 Canadians, 12 Progressives, and 8 non-affiliated, which last category raises interesting questions about the meaning of "Independent".)
It's worth noting that the American two-party system is explicitly protected by many laws that grant special privileges to "the two largest parties" or some similar category; it doesn't rely on the electoral system at all. If other parties got equal treatment before the law, you might see more of them around.
The last time Canada had anyone other than conservatives or liberals govern the country was more than 100 years ago. [1] What kind of opposition is warming the rest of the parliament seats doesn't matter all that much, because just like in the US, there is no culture of inter-party collaboration in Canada, because there is no need for it: minority governments are rare (thanks to FPTP). This is very much unlike countries with proportional representation, where coalition governments are the norm, because when stupid voting mechanics are not protecting the duopoly, no single party is ever good enough to capture more than half of the vote.
The senate in Canada isn't even elected, senators are appointed by the Prime Minister. And it doesn't have the same prominent role as the US senate. So I've no idea what are you trying to say here.
FPTP favors the two biggest parties by a huge margin. It's damn math, it's been proven decades ago. Look it up.
[1] https://en.wikipedia.org/wiki/List_of_Canadian_federal_gener...
New Zealand has used MMP for 25 years and yet it still dithers between two major parties. Minor parties do tend to have to be incorporated into coalitions with one or the other major party, which is something, however, they’re almost always the same minor parties falling along the left right divide. Electoral systems do have an effect and they are setup to make it hard to radically change the system, however, they strengthen they don’t create the two party dynamic. Two major parties aligned with common human biases always seem to spring up when people are free to choose.