So what, they have to change their name if they ever want to branch out?
Why don't marvel studios make some Batman movies?
The reason that 343 are such a talented studio is because Microsoft reached deep into its pockets to ensure that they had the best talent they could get to ensure that one of videogames's biggest franchises remains of a high standard.
And to be quite blunt, there is no evidence to suggest that they would be the best studio to handle something outside of a FPS.
Just like there is no evidence to suggest that Naughty Dog would be Good at making a serious racing game.
It's hard to know what is the right thing to do because you've got the opinion that 343i are garbage!!111 and can't even make Halo games. Simultaneously you've got people saying 343i are super talented and Microsoft have wasted the studio.
Then you've got people who like what 343i have done so far.
It's hard to know which kinds of comments to even take seriously due to many individual's predilection to Neg everything Microsoft say or do, which appears to preface many opinions.