I think we are talking about the same thing, in essence.
-First off, the endings A and B are poorly written and even more poorly implemented. They play like afterthoughts and cut & pastes more than actual endings.
-Both are anticlimactic. While my main save was the C Ending, as I believe it is both canon (as you suggested) and would have been prerequisites for DLC that never happened. I had no idea back then that this would be the case. But even so, I left a save slot in that crossroad mission so I could re-load it and go back to play the two alternate endings. While I had zero interest in killing Trevor or Michael in my game, even back then, I had still assumed that their executions would be better thought out and better developed, not to mention better written. I actually really looked forward to playing them, even if they would not result in my final save point. But in the end, the missions are basically some dumb "chase the guy down, drive a long way to some destination, and kill him." Very disappointing from both an alternate ending standpoint AND a gameplay standpoint.
-The problem with a game that offers only ONE good choice is that, in my opinion, it devalues the whole point of offering the player a choice in the first place. I mean, shouldn't the devs and writers have offered players an actual choice between three viable endings, rather than one really good one and two completely dead end (your words) crappy ones? Again, if I were a writer or dev, I would want to make the player really think hard about their choice, by incentivizing the two "not as happy and Sunshine & Puppies endings." Maybe I am overthinking this, but I would have wanted the players to really have to sit and think a minute about whether they truly wanted to let all protagonists live OR wanted to kill one of them. As much as I hate IV, this is one area where I think the story choice is, in effect, better than the fork in the road for V. Because, if I remember correctly, choosing either ending gives the player both advantages and disadvantages. Meaning, you really have to think (assuming you care more about the story than I did, of course) about what ending you wanted for your final save. You were both rewarded AND punished for either choice.
-I am separating story from gameplay in one sense, but even in terms of writing, both the A and B endings are awful. Forget about the effect on the game, for a minute. They are delivered in a very generic way where the writing is pretty much identical, except for in one instance you let the guy drop and in the other, you set him on fire. But everything else about the writing in these ending choices is nothing more than written cut & pastes, and gameplay cut & pastes as well.