He didn't like fanart? Are you sure?
Yes, as far as can be told - as I said before, he was flattered, but he thought the effort would be better put into creating something new and original.
I would find that out of Hergé's character.
Why would or should it be out of character? He didn't ask for it, so his opinion was entirely his own.
Are you saying he would throw away their fanart?
You are putting words into my mouth there, which is going a bit far; I have no idea what, if anything, he did with it.
I find it kind of rude if he never appreciated the kids or adults' fan work.
Why should it be "rude"? If I sent you something you didn't want and never asked for, would it be rude of you to say you didn't want it and never asked for it? It's simply the truth.
Actually, if you've read what I wrote on the subject before, he did
appreciate the effort - he would often write back and thank the sender for sending it: but usually with the advice that they did their own thing in the future.
Advocating what? I would love more Tintin
"Advocating" means speaking up in favour of something. You seem to want more Tintin to be done, and to be saying that it should be done, while still maintaining that the wishes of Hergé should be respected. To me you can't ever do both.