I've heard it suggested several times on this site that advisors (especially in experimental fields where the PI may benefit from a cheap, highly skilled workforce) have an incentive to prevent their PhD students from graduating in a timely manner, or at least that they have no incentive to actively promote graduation when the student is "ready." (Whatever "ready" means for that particular student.)
For example, user47148 says
You are the only one who cares if you finish. To your advisor you are cheap labor.
I often wonder if keeping highly-skilled cheap labor around to do something with the data that a PI is collecting from a high-profile grant is often an implicit factor that impacts students' trajectories. In which case it is not uncommon to see students in their 6th or 7th year of the program receive funding.
keeping your advisor from keeping you there forever as cheap labor.
I am interested in learning more about the opposite: what incentives do advisors have to help their students graduate as soon as they're ready?
This question discusses incentives for advisors to increase their PhD completion rate. Presumably this is part of the answer, because a student who isn't allowed to graduate in a timely manner might just drop out instead.
But besides for that, what incentives (if any) does an advisor have to help students graduate when they are ready, rather than dragging out the length of their degree?
(In some programs, there are policies that limit the duration of PhD funding. I'm asking about programs where PhD students can potentially hang on for several years past the mean time-to-degree.)
P.S. answers supported by references to actual data would be amazing.