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It’s expected and it is normal.

There is no term limit for PM or members of parliament.

They stay on until they lose the support of their party.

He lost the support of his party due to his extreme unpopularity and the impact it will have on the future election. As seen by polls and bye elections.

More often the leader loses party support after an election loss.

However in this case, a loss is so likely and expected to be so bad that his party would rather go to the polls with a different leader.