Music is moving slowly, its 30 most recent threads include ones that were last active in early 2019. For Sports it's even slower because the section is mostly focused on megathreads that encompass an entire season of football, NFL, NBA, NHL or MLB, so discussions are mostly contained within these few threads. Movies is the only subsection with a decent number of different threads because TV shows fall within the same subsection.
It's hard to imagine that anything important would get lost in the shuffle if these three subsections would get combined into one larger Off-topic subsection. The frequency of new threads is low for all three combined, the megathreads of the sports variety will see regular bumps because there are games played every week.
As for that mini-screenshot of the new frontpage that is in the works, you would reduce scrolling to the sides a lot more by using drop-down menus for Gaming, Website and Off-Topic. There are only three main sections, making sideways scrolling redundant. With the arrows on both sides being removed, you'd have the necessary space for three options being displayed next to each other.