Rare didn't need replacing, they hadn't made a quality game in years and all the people responsible for their quality titles had left by the time MSFT came calling.
I agree though that Nintendo needs to do more to replace SK. They've done some expansion in house though (added a new 1st party development team), so that's something. Personally I'd like to see them add a few more in house teams, that's the best way to ensure you don't get in a bidding war with MSFT over a studio. I'd follow the consolidation trend of the industry and get ahold of somebody with some valuable properties and some proven development talent, like capcom.