Personally, even dwelling on the thought of a technically "unbeatable" final boss makes my stomach churn, knowing that fighting it is for nothing.
Well, at least for me, it's not "fighting for nothing" because I still personally experience the ending, even if my save file cannot "save" it. As long as I can still save my stats/completion percentage I'll be happy.
Lastly, as for the example of DKC2, the ending would have been different if the player had been able to save after it.
But I think that would have ruined an awesome ending for the sake of not being able to refight the final boss. Actually, now that I think about it, DKC2
does save after the final boss, but it only saves the stats, not the story, so you can replay any part again. I guess for me I just want games to have a replay option, either in the main menu or in the original file itself, so that I can relive my favorite moments without having to churn through the entire game all over again.