I don't think the odds favor Iraq.
Between the end of WWII and the Iraq invasion, there were about 122 civil wars. Six of these civil wars have been resolved by power sharing (3 of these are a little dubious - where the civil war was considered ended since fighting stopped for 5 years, but was followed by a new civil war around 10 years later).
'About' 122 since it's sometimes hard to decide whether a country had one real long civil war or two separate civil wars. If several groups of insurgents band together to win a civil war, but then their alliance falls apart and they start fighting among themselves, it's generally considered a new civil war; not a continuation of the first.
Afghanistan would be an example - insurgents won the civil war of 1978-1992, but a new civil war broke out among the different groups within the winners. The Taliban, with an influx of money and aid from Al-Qaeda, finally gained the upper hand, but they didn't stay in power enough to really say the post-1992 civil war has really ended. In spite of a new democratic government, you'd probably have to say Afghanistan is still fighting the post-1992 civil war and not a new one. It's still the same parties fighting each other.
Coups and revolutions make for short civil wars - a median duration of around 2.5 years with a mean of around 3.2 years.
In 'sons of soil' type civil wars - where the civil war resulted from one side exploiting the resources of ethnic minorities (or majorities), civil wars have a mean duration of 32.4 years and a mean duration of 41.4 years.
Iraq would fit this category if civil war had broken out on its own without a US invasion. It still has a lot of the traits of the 'sons of soils' type of civil wars, but maybe it should be lumped in to the 'all the rest' group, which has a median duration of 10.3 years and a mean of 13.2 years. Personally, I think the pre-invasion Sunni dominance of the Kurds and Shi'ites carries a lot of baggage that makes Iraq closer to the 'sons of soil' type civil war.
The six cases where power sharing successfully resolved the civil war:
Lebanon 1958 (one of the dubious successes - the success didn't last)
Sudan 1972 (one of the dubious successes - fighting stopped for 11 years, but then an even bloodier civil war broke out)
Zimbabwe 1979 (another dubious success - a new civil war eventually broke out)
Mozambique 1992 (successful resolution of a civil war fought for political reasons, not ethnic)
South Africa 1994 (successful because of Nelson Mandela - he truly is one of the great leaders in human history)
Guatemala 1996 (a second successful resolution of ethnic civil war through power sharing - Edit: successful at least as of 2002 - it still seems to be a mix of very serious problems and some very promising hopes)
Every other civil war was resolved through one side winning a clear victory or in a stalemate imposed by external parties. Bosnia, for example. If peacekeepers leave, civil war breaks out again almost instantly. There's been no resolution even 10 years later.
Sistani hasn't turned out to be another Nelson Mandela, so the most likely successful outcome for both Iraq (and Afghanistan) is peacekeeping succeeds and succeeds for the next few decades? Or is the most successful outcome to have Sunnis, Shiites, and Kurds fight it out until one side finally wins? The last method is usually the best hope for a long term solution.