I have no statistics for you, but I think the rate of failure will be over-represented in any forum. People tend to post when they have problems or when things aren't working out. People who have been in the same, stable multi-partnered relationship for decades have no reason to join or post. People who do join and/or post do so because they have questions or problems, and obviously when there are problems, sometimes a relationship ends.
If anyone has statistics, I would be interested in them as well, but I think it would be difficult to get them. Many people would need to be followed for many years to get any results that are relevant and helpful, and this kind of research is expensive, and not likely to be funded for a subject such as polyamory, which only concerns a subset of the population.
The statistics are more likely to come from surveys, etc, which means there is a bias from the starts, as only the people who find and take the survey are surveyed, not the general poly population.
And for any figure to make sense, it should actually follow the general population, poly or not, so that one could see how many people switch from one to the other and how many people stay with the same relationship model, and which lasts longer, etc.
It's also important to differentiate between poly people who are single or in a couple (therefore not with multiple partners), vs people who are monogamous, since your specific query is about a change of lifestyle, and not finding a partner isn't the same thing as not looking for one or actively rejecting the idea of having one.