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
Specific examples of this are found in the 'rediscovered pages' released in the newest Egmont edition. Page 57 documents Hergé's thought process as he tried to work out an escape for Tintin!
Will he hide in the ceiling? A hole in the wall? Wear through his bonds? Have Snowy chew through them? Hergé considered these options, and who knows how many more!
This strongly indicates that Hergé had no intention of displaying Tintin's final grim resting place, the life-size polyurethane-cast sculpture, in his studios along with Sir Frances Haddock, the Carreidas 160 jet, and the moon rocket, to remember his late hero by. (A very long sentence!)
Mr.Remi could have been in a lot of pain in the days before he passed away, so I could see a despondent Mr. Remis writing such a book where Tintin has his final adventure.
As Jock123 so aptly points out, Hergé was apparently in relatively good spirits before his death.
It's an elementary mistake to make, but Hergé wasn't writing the book in the last days of his life - he hadn't done any work on it (as far as can be told) for a few years before his death
There is nothing to suggest that he was despondent or down-cast in his work toward the end
However, even if he was courting the blue devils during the writing of Alph-Art, or before his death, there is no indication that this would have produced a more tragic tale. Tintin in Tibet was the brainchild of one of Hergé's more notable periods of depression. It is very interesting to appreciate how deeply the story dives into the bleakness of depression, loss, pain, but ends on the triumphant chords of friendship, love, hope. Who's to say that a Hergé in pain would have put an end to his Tintin?