In the past I would have been 'what the hell' - but I've seen what happens to such things after authors and artists pass on - they're removed from the process, obviously, of bringing the piece into full flavor and fruition and people just dive in one it like greedy money grubbers.
Musicians have it a bit worse. Some of my favorite bands are constantly being churned over and over like butter for the last few pennies the family can wring out of their legacy. It's vile.
A danger of being successful, I guess.
Sir Terry was a wise man in this case (and many others). One's literary legacy should not be reams of posthumous works completed by other authors, esp. ones that you, yourself decided not to finish for whatever reason.
On the up side, those needing a Pratchett fix have the Good Omens TV series to look forward to. The writer and showrunner is Neil Gaiman, who co-wrote the original novel with Pratchett, and a company owned by Pratchett's family is backing it so there's a good chance this will actually be done right.