Future will tell
Indeed the future will tell. The main thing for me is that as a ladder or league to add an expansion to the default maps etc. you need a consent of most of the clans playing with you. Just like adding a new map to a ladder or league.
Even if only 25% of the clans do not want to play RS, you're at the issue of adding RS and losing 25% of your clans or staying stock with some custom maps and keeping the entire population. Especially if RS might feel very different to RO:HOS some clans might be against it.
In the end its the communities that primarily decide what they want to play, and history tells us that paid expansions are hard to push through in the same ladder as a general league.
In case of the original RO:Ladder we pretty much couldn't offer multiple competitions for the mods for instance, something that the new ladder will solve. But its the community that decides how certain ladders are filled in.
Early on in a ladder most clans are still learning most things anyway and are more flexible in seeing changes in the ladders or leagues. But once a ladder or league has been going for a while, people begin to act more conservative.
Which is why for instance with BC2 the vietnam expansion got separate ladders, than for instance BC2 itself.