It's not like IG makes RPGs, so why would they need to get an RPG developer to fill the "void" left by Insomniac going full time multiplat with EA? That should be expected btw, the 1 game deal they have with EA now is probably a test of sorts to see how the game will sell on the 360 and see if it's worth the extra resources and double the QA work. Considering they'll probably make a shooter, it'll probably do well enough for them to go full time with their EA partnership.
As far as platformers go, Naughty Dog, Media Molecule, Sucker Punch, Read at Dawn have all proven to make as good, or better, platformers than Insomniac makes; so SCE shouldn't be worried. If anything they should look into acquiring SP and RaD, because those two, along with Clap Hanz, are the companies that make the most sense for SCE to buy, if it is looking to expand its WWS.