The show has already been cancelled. NBC is now 'burning off' the remaining nine episodes in the summer months. Highway to Heaven will conclude its five year run on NBC the first week in August.
Victor French had lung cancer at this time, but the public did not know that. It has been speculated that French's illness and death contributed to the cancellation of the program (French died just a few weeks later in June 1989). However, it seems that NBC was not happy with the show's performance and that is what caused it to be placed on hiatus three times and to eventually be cancelled. If the ratings had been stronger and the show had been picked up for a sixth season, it seems inconceivable that Landon would've soldiered on alone without French. The format may have changed and a new sidekick possibly introduced. We know that if given the chance to do an episode about the death of Mark, Landon would've created a tearjerker episode to end all tearjerkers!
The show is now on Fridays.