Coudn't Herge have made Alph-Art Tintin's last adventure without actually killing Tintin?
Of course! Nothing
that is in the published notes, the work in progress or anything else which has come down to us from Hergé shows that he had any intention of killing off Tintin in this book – it's purely speculation
by others, based on the fact that it is de facto
the last story upon which Hergé worked, and that at the point it appears to end, Tintin is apparently in peril.
However, the book is only a very rough outline of ideas that Hergé had had, and it is entirely possible he'd have re-worked it, revised it or even abandoned it if matters in his own life had taken a different turn.
He certainly set the book aside to work on the designs of the murals for the Stokkel Metro station, which was actually the last proper Tintin project he seems to have worked on, and that doesn't hint at anything like the idea that the character would have actually "died" in a story.
He also had research in train for stories returning Tintin to the lands of the Native American peoples, and an interest in sending Tintin to Australia (this last had a considerable amount of material in his reference collection, according to Michael Farr).
So there could easily have been life for Tintin after Alph-Art, had Hergé himself lived, and been interested in doing another book.