mct16:
But these books were published within a couple of years after Hergé's death. They must have been a long term project that had been planned for some time.
I'm not so certain - I think there was a feeling amongst the people in charge at the time that the Studios needed projects to keep going, and this and the continuation of work on
Alph-Art were the ones which came to hand.
We are also still dealing with what did - and didn't – happen, post Hergé, regardless of proximity or otherwise to his death: the on-going, long-planned,
Alph-Art with Tintin didn't get worked on, and the non-Tintin Quick & Flupke didn't.
If Mme. Rodwell didn't want to come out with new Tintin stories when they still had the capability of doing it in-house, it seems even more unlikely when that capability has been (deliberately) dismantled.
mct16:
Belgium can't lack in studios
Well of course, but you are missing the very important fact that they aren't
Hergé's studios; they'd be taking the work into someone else's domain, which would be a very different kettle of fish.
Technique is the least of the problems - you could go to any country in the world and find artists capable of reproducing work in Hergé's style - or anybody else's style, come to that.
What you wouldn't have is the hand of the master in charge, working in his chosen environment, with his carefully assembled, time-tested staff to hand - that's what makes a studio, not just the paper and desks and chairs.
mct16:
What if they were to make more illustrations using the computer-generated characters but basing it on the ThermoZéro storyline?
Of course that would be possible - but why would they do it? The one thing we know absolutely for sure is that Hergé dismissed the story as a Tintin project; under the circumstances I'd guess that that would be seen pretty much as the definitive statement on the matter, so it would look like a no-hoper to me. Not impossible, but indicative...