We had the same shit in New Zealand.  15 years of stable government (well, barring Winston Peters, but even he didn't upset things too much - National still went full-term) has very much proved them wrong.

Instability is a product of political culture.  And voters can change that culture by de-electing those who do not play along.  If the UK got PR, its politicians would have to unlearn the bad habits of FPP, and learn instead to consult, negotiate and compromise.  They won't like that - but the choice shouldn't be up to them, it should be up to the people.  They dance to our tune, not the other way round!