Referendum or voting in their representatives. As an example, assume 80% want to introduce racial segregation in each of the UK, Australia, the US and Canada. What measures would stop them?
For example, the potential election of candidates within a subset that escapes that 80%, as enabled by systems similar to those listed, which have exactly the purpose of approximating optimal choice.
None of the systems you listed will protect against an 80% majority wanting to force through specific legislation. Through brute force they will get their way eventually and inevitably.
If you disagree, I hope you can give a more fleshed out example than, what seems to be, to be the vague handwaving you have been doing so far - no offense intended.
> None of the systems you listed will protect against an 80% majority wanting to force through specific legislation
Close to impossible to say without consideration of specific implementations of the exemplary sub-frameworks proposed.
> a more fleshed out example
A John Rawls like framework of common best interest in not appointing beasts, coupled with criteria to discriminate beasts? The point is not about providing a detailed solution here, but in pointing in a direction beyond simple, lacking forms of democracy, suboptimal in theory and in results. If you have naïve processes in place, faults will eventually show: the system must become resistant. A large number of possible improvements can be considered: the point remains that they must be studied - this has never been the "end of history", if only because in too many systems the fool and the bright have similar impact.
And I need to say: like in all systems, checking what went wrong, why it went wrong, and implementing solutions so that it will not go wrong again.