Re: I wonder
I think he was right to ask it, and I say the absolute proof of this is the Sky TV adaptations.
David Jason is one of my all-time favourite actors (Dangermouse is still his best work, incidentally).
But David Jason is also the quintessence of institutionalised "national treasure" -- they dragged Only Fools And Horses out well past its sell-by date, and made it all soppy and sentimental, and then when they had finally cancelled it, they brought it back to give it a happy ending twice, completely against the core point of the original concept. People kept watching it even after it became essentially unwatchable, simply because it was Only Fools And Horses, and in their heads it was "great". And the "national treasure" pull was so strong that they even made that god-awful young Del-boy series Rock and Chips, which was so meta in its double-nostalgia it made your head spin.
Then there was A Touch of Frost. It was a fantastic series. They had a great lead character, well acted by Jason. He was a bit old, but it was fine. They kept convincing him not to retire, and talking him back into the studio for "one more series", until we had a TV detective a good decade beyond retiral age, who presumably needed a stunt double for any run of more than a few metres. But we can't cancel A Touch of Frost, can we? It's a national treasure! And the writing and direction got really, really crap towards the end. The second-last series was so dire I'm surprised they even bothered to commission the last one. And the last involved some really sad stuff in the run-up to the finale, yet they still painted it as a happy ending, because national treasures always need a happy ending... even if that happy ending is at the funeral of their best friend. Shoddy, shoddy writing.
So that brings us back to Discworld. Rincewind was the wizard who ran away from everything, so 68-year-old David Jason was not the right man for the character by a long chalk. The only reason to include Jason was... he's a national treasure. And that still wouldn't have been enough if Discworld hadn't been... a national treasure. People watched it because... national treasure.
If his writings had survived, that whole national treasure thing would have led to loveless stringing out of the series to satisfy our lust for national treasure.
The very symbolic way in which it was done also cements in fans' consciousness exactly how much Pratchett is against that, and kills the commercial viability of national-treasure-pot-boilerism, because many of them would see the writing of a non-Pratchett Discworld novel as betraying their man.