The ending (spolier alert, although I'm sure you've all seen it)
I saw this over 30 years ago, and forgot the downer ending. The effects are pathetic to say the least, and most of the cast is wooden, but it doesn't stop this from being a very unpleasant viewing experience. The three guys left for the third act are very likable. GET THIS THOUGH, Grizzly follows Benchley's original novel's ending more closely, two out of three on the Orca die, and the shark gets away. Here the bear is almost bullet proof at least even if it does get blown up. In Grizzly you get a somber, down beat ending and no dramatic uplifting payoff. This is something it gets right, where I think Benchley and Spielberg dumbed down the script for Jaws.
I've seen an ending like this before, in a movie that does it far too well, Quatermass and the Pit, of all things. James Donald sacrifices himself to stop the monster there, with a dark somber ending, even though he goes down with such a determined look on his face. the movie That film was made in 1969, and I saw it on TV in 1976. Maybe I'm just too Gen X for my own good, i.e. too cynical to appreciate a happy ending.
The ludicrous explosion aside, Grizzly upped the ante with its downbeat ending.
I don't think this is much of a stretch, do you?