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.
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.
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...