If ICE has Shadow World IP control and wants to reuse it, then maybe change SW to fit with the RMU Core, Creature, Spell, and Treasure Law stuff.
No.
Setting is more important than rules.
Yes, setting is more important than rules. Rules are just names, things, and potential actions. Setting adds conflicts, issues, locations, structures, events, history, and lore—the description, specifics, and details of those potential names, and things.
Hence, the hopes for a RMU default world that fits.
If anything, the RMU rules should be hacked to fit the setting to a T (which the RM rules never quite did).
I suppose that is so, only if
Shadow World writers and creators are quickly willing to adopt and use the new version. If not and nostalgic mentality prevails, then even
Shadow World might become a relic of the past—ICE's legacy setting similar to "The World of Greyhawk", replaced by whatever their new setting similar to "Forgotten Realms" or "Eberron" becomes.
If some major SW writers and creators don't fill the void, then move on from it.