A potential client today told me that less than 50% of those enrolling in Ph.D. programs complete their degree, and most of those who do not, fail to do so because they do not finish their dissertation. I don't know what sources she had for her claim, but this seems to be in line with what I can find online--for example, the report on Ph.D. graduation rates by the University of Maryland, Baltimore County, Office of Institutional Research. (Updated: this article puts rates at between 50% and 66%, depending on field of study.)
And that's a terrible shame for those who have invested their money, time and effort only to fail in the final hurdle.
But those statistics are not predictive: no individual is doomed to this failure simply because so many have gone astray in the past.
Getting help and direct support can be key. And even if hiring someone--an editor or coach--is expensive, it may well be worth it in terms of time saved, increased future earnings and decreased future tuition/registration costs.
One potential client who called me had maxed out all her possible student loans and still was looking at another year or more of work. But what does she do now? The key is always to find the most productive response to the situation you face.