[0]: https://en.wikipedia.org/wiki/Duverger's_law
[1]: https://en.wikipedia.org/wiki/Australian_Electoral_Commissio...
+ liberal democracies have (at least in theory) unalienable basic rights that can't be taken away, even by majority. Eg just because a 90+% majority votes to revoke citizenship from all muslims that doesn't mean that it will go through.
As for these "inalienable" right, I'm not aware of any system without a mechanism for altering their constitutional laws.
To your specific example, the UK just had a government representing a minority of voters vote through a law that allows them to strip citizenship from millions as they please.
we're not talking about some ambiguous grey zone stuff here
* proposed a drastic reduction in immigration with particular reference to immigrants from Asia.
* stated that African immigrants were bringing diseases into Australia and were of "no benefit to this country whatsoever"
* called for a ban on Muslim immigration to Australia
* announced policies including installing CCTV cameras in all existing mosques
* stated "all terrorist attacks in this country have been by Muslims"
it's safe to call the above extremist and xenophobic
A huge danger of systems that downplay the support for extremist parties is that they create an incentive for infiltrating the bigger parties instead.
I may not agree with the politics of One Nation (in fact I'm almost certain I don't, without even going to read what they are) but having a system where I can place a vote for what I really want and then put an acceptable compromise candidate as second choice seems to me a much better way of allowing newer parties and newer politics in than the more or less institutionalised stagnation of something like the US system.
From what I can tell the Australian system rarely lets such smaller parties gain very much anyway.
When you have 150 single-member districts you're probably going to get two major parties regardless of whether you have FPTP or IRV or even Approval voting. What compulsory voting and preferencing add on top of that is both of Australia's majors have to win the centre, rather than energising their base per the US.
What our system does allow for is for smaller parties in our Senate. It's generally elected 6 people at a time, allowing for e.g. 1 Green, 2 Labor, 2 Liberal-National and 1 One Nation (this is a very plausible result for Queensland next year).
So what does the US system allow then? Well, every system has to let its steam out somewhere. The US has its primaries, which often has a level of access and openness that greatly surpasses any Australian preselection (generally still getting out of the smoke-filled-room era).
I like the idea that the main parties have to capture the centre, and I really like that I can vote for something a bit out there, without the fear that my vote is effectively a vote for my least favourite big incumbent. The "You must vote Labour or the Tories win!" thing has always rubbed me up the wrong way, as someone who isn't especially fond of either of those options.