Yes, because creating a new studio for the sole purpose of continuing to provide the core userbase what it wants isn't serving the core userbase?
I wouldn't be at all surprised if the cost of creating the new studio added to the cost of developing Halo 4 exceeds 100 million USD (pure guess, no sources). If that is abandoning the core gamer, then...uh. What?
And unlikely spending 100 million USD on a new IP, MS are likely quite convinced they'll recoup their entire investment and then some because they already know that the core userbase will buy it in droves, because that is what a large chunk of the core userbase is demanding.
Creating new IP isn't bad. It's risky certainly, and could sink your company, but it's not "bad." And certainly giving your userbase what it is demanding most certainly isn't bad. So what's wrong with giving the core user what it demands? Especially when there is little to no evidence that consumer fatigue is starting to set in for the franchise?
Would you say the same thing about Sony if they had announced no new "core" IPs (whatever that is as it appears to move depending on the user) and instead just announced sequels for GOW, Uncharted, Resistance, and Killzone?
And in the process showed off 2-3 new IPs for Move with a casual orientation?
Regards,
SB