I used to think the same, then I realize that topic is just a concept we draw from the content. It just reminded me the time we do feature engineering for many things, now we just feed everything into the algorithm and let it figure out how to handle things. Maybe the problem for us is not how to organize data in a way we think it is best, but for a format which is useful for the algorithm. A fixed window size fits perfectly with the embedding model would just work.