In narratives, the Protagonist almost always gets stronger.
The most obvious examples are most popular Anime, where the more they get beat up the bigger attacks they do, but it's also the case in films, novels and most other stories.
i.e. The shot a Protagonist couldn't make in training at the start of the film is garenteed to hit when he makes it at the end while injured and stressed.
If anything the normal D&D way of getting weaker the longer you fight, while realistic, makes for weaker climax scenes because you don't have the resources left to deal with the final encounter if it's too big.
Lord Kiwi,