Earlier quoted context omitted.
This is how planning should be done. Gauge people's expertise, put them on the path they can be most effective and then ask for their estimates. Take that to upper management and negotiate.
One problem with that is sometimes the estimates are done long before the work starts so when that item eventually gets to the top of the priority stack the person estimated for is working on something else instead. At this point things need to be re-estimated which can cause friction if the new estimate is longer (“but last time, you said…” or “when we asked Bob, he said…”). What I've done in the past is give an est…
I like to think of it like Algorithm Analysis; Best-Case (estimate by the expert mentioned above), Worst-Case (estimate by Noob who just joined the project) and Avg-Case (estimate by person who is already part of the project but is not responsible for that module). Add a fudge factor based on your reading of the circumstances and then haggle with Management. You now have a band with a upper bound and lower bound which can be realistic.
PS: You may find Douglas Hubbard's How to Measure Anything: Finding the Value of Intangibles in Business useful in this regard.