The Labour Party has governed for only 21 years out of the 80 years since the First World War.
If the electoral system is not changed now, there may not be another chance for a generation. Like the weather, the electorate is changeable.
To assume that because Labour has a large majority now it always will is as foolish as to assume "bright confident morning" will last all day. Not to change the electoral system while there is the opportunity will mean more Conservative governments.
Nor will spurning the Liberal Democratic Party and its present leader assist Labour to win elections. After the Labour landslide of 1945 the Labour vote held up well in 1950, but it lost its effective majority, arguably owing to a fall in the Liberal vote. The same could be true if the next election is close. Labour still needs the LibDems to do well in the south and west of England to stop the Conservatives.
Whether Tony Blair wishes to govern from the centre-left, the centre, or even the centre-right, he has a much better chance of doing so under some form of PR.