It does occasionally happen, though. e.g. in Britain Labour replaced the Liberals in the 20s and 30s. Conservatives got almost destroyed last election if polls

It does occasionally happen, though. e.g. in Britain Labour replaced the Liberals in the 20s and 30s. Conservatives got almost destroyed last election if polls stay were they are they will likely be replaced by Farage's party. France maybe doesn't fully count but when turnout is very high they are almost FPTP. Also the second round when it happens is usually three way and sometimes even four way. And their party system is about as hectic as it gets (i.e. most PR countries in Europe are much more stable)...