Honestly, I don't even think there should've been another rocky film period. However, in context to the story, I think it turned out way better than a lot of people expected; including me. I think in the end, writing her off in the film only helped make Rocky Balboa more sympathetic in the last movie. Giving him more of an emotional hurdle to overcome, as they made it obvious that his son was already moving on, or trying to, without him. His wife is dead, so all he has left is memories of his past that bring him joy, but also sadness from how he knows that he'll never see her again. I think it works better for the story versus if they would've kept her alive, and he just wanted to come out of retirement for the pure hell of it. That would've been moronically stupid and cliched. No, I think writing off the character, while giving Rocky another emotional hurdle to over come, only gives the movie a lot more emotional weight that was missing in the last two films.
Although in hindsight, I think they could've wrote her in the beginning of the film, then have her die like halfway or twenty minutes into the movie to add dramatic effect, but that's just me.