Nothing if that's what would actually happen. If you believe that would be the outcome then you have more faith than me in our politicians.
I see a system where you end up with either labour or tories permanently propped up by fringe parties (which we would see more of), and it impossible to get rid of disgraced MPs because the party still keeps chucking them on the list somewhere rather than not standing them when they're being specifically voted for.
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And it was no doubt similar when labour were running things, but when they pissed off the country enough enough seats changed hands to change the government. Not forced to change as much with PR.