What caused the finish coat to separate from the brown coat plaster ?
I have worked on many plaster walls & ceilings that were any where from 200 years to 150 years old & the finish coat was not separating, but I have seen a lot of drywall mud over plaster fail. My thinking on the OP project is that the new applied Joint Compound over the plaster substrate will have cracks at the edges or the existing skim coat edge material will separate I give it 60 days after it is completed.
the plastering industry would still be a vibrant industry.
However its not. Its dead.
The reason for the decline in the Plaster trade was due to the invention of Drywall. The drywall was cheaper in cost less labor to install / apply is the reason for it's decline. As for the Plaster trade being dead that is some what correct due to the skill required to apply Plaster & make it smooth.If that was even remotely true, the plastering industry would still be a vibrant industry.
However its not. Its dead. Instead, PAINTERS are patching PLASTER with drywall compound, and CALLING it plaster...
And 99% of the time, it holds..... FOR DECADES...