Mission creep itself is not a myth. Britain's 'accidental empire' - at least if standard accounts are correct - is an example. Britain's original 'mission' was to establish trade outposts. But the British got involved in Indian politics and, one thing leading to another, became a full-out colonizers. So their mission gradually changed from establishing secure trade routes to founding an empire.
Here's another example: the US entered World War II with no further aim than to defeat the Axis powers. When victory was in sight, the mission expanded into remaking Germany and Japan. The expansion of objectives is a case of mission creep.
You will never hear either of these cases *called* mission creep. That's because they were successful. The term isn't normally used to describe a real-world phenomenon. It's used to explain away failure, and, unsurprisingly, this almost always involves some myth-making.
The example of Afghanistan has lessons which also apply to Iraq. Despite all the fine talk of establishing democracy and such, 'the mission' in Afghanistan was straightforward: to crush anti-American forces. Not to nit-pick: if you want to say there was a broader mission, fine, but it certainly was not the case that American policy crept towards it: in fact it crept in the opposite direction when the prerequisite for any noble objective - control of the territory - proved elusive. That the US attempted to do too much was not the problem. 'Mission creep' was the lame excuse for failure to accomplish the minimum.
That failure was due not to over-ambitious idealism but to politically induced deafness. Military experts were quite clear what the mission - the minimal project of establishing control - required. They said it would require about 650,000 troops, a commitment Washington could never sell to the country. The decision-makers got around this mental roadblock partly by simply not listening, partly through the absurd fantasy that an indigenous Afghan army would do much of the work. Apparently Vietnam was forgotten. As a result and as the expert estimates implied, the US never established military control of the territory. But the US could not acknowledge its mistake because that would highlight its very dangerous weakness, its political inability to mount the effort required to conduct a major military operation overseas. So it engaged in all sorts of blather about hearts and minds, and, of course, mission creep. But hearts, minds and missions required more men and more guns and a lot more casualties. That, not made-up subtleties about The Nature of Counterinsurgency, was what most Americans never grasped.
The same, long story short, held for Iraq: the US never committed anything remotely like the military resources required. That was why, as in Afghanistan, it devoted astronomical sums to various (roughly speaking) 'reconstruction' projects. This supported the Iraq-specific excuse that 'we won the war, but we could not win the peace', that we were unprepared for this additional task. But it takes no deep thinking to realize you can't reconstruct - and indeed you haven't won the war - unless you can assert control over the territory. This, again, required more troops and casualties, and was known to require them, than the US was politically able to afford. The same weakness led to the same excuse: mission creep.
No one, I think, anticipated the cost of an ever-deeper attachment to these excuses. To raise a false spectre of mission creep doesn't just cover up ordinary failure; it paralyses even very manageable undertakings. In Syria, effective action requires no troops and very little expenditure; it is simply a matter of supplying some of the non-US fighters. But the US cannot even do this because it has instilled in its population - indeed in its pundits - the fear that somehow, somehow, there is a mission and it will creep. It didn't make sense before; it doesn't make sense now. But people are used to the idea, and it stokes their anxieties. As a result, the US is politically - and therefore militarily - even weaker than in the previous decade.