Both in 2005 and 2015, incumbent Prime Ministers went into, and won, General Elections on the understanding that they would be standing down at some point during the subsequent Parliaments.
There is no comparison with the overthrow of Margaret Thatcher against her will, or with the sudden resignation of Harold Wilson because, as we now know, his health had started to fail.
Twice in very recent years, then, we have seen the choice of Prime Minister ceded in advance from the electorate, or even the Queen, to the internal structures of the governing party.
Both main parties have now done this. The practice calls for further examination and evaluation.