You make a very good point about the nascent extremism - as primaries become the only elections that matter, and because you need extreme candidates to flip those locked districts, there is little reward in moderation. I find it funny how Republicans embraced extremism while Democrats seem to be more averse to it, even though they'd have much to gain in this scenario.

This is what I really like about Ireland's elections - they are so effective in weeding out extremists that the two dominant political forces are almost indistinguishable. It's, perhaps, too efficient in that, but it also seems to be extremely safe and long-term stable.