I get your concerns, but this IS an instance where I think progress over perfection is key.

The mere idea of changing a voting process is a big hurdle, and if it is shown that "yeah, we can do that" and that FPTP is not the only voting system in history, we have achieved a victory.

And as others have said, IRV is a huge win by allowing people to vote "FOR" something, rather than strategically vote against something.

So, even if the third parties rarely break out, it would more accurately poll the public's sympathies.