Note that multipolar scenarios can arise well before we have the capability to implement a SI.

The standard Hansonian scenario starts with human-level "ems" (emulations). If from-scratch AI development turns out to be difficult, we may develop partial-uploading technology first, and a highly multipolar em scenario would be likely at that point. Of course, AI research would still be on the table in such a scenario, so it wouldn't necessarily be multipolar for very long.