They're just taking their time, as they should. Halo 5 was, generally, an improvement on 4, but still nowhere near the heights Bungie reached with the series, especially regarding the single-player design. So fingers crossed the third time is the charm.
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343 was literally formed for the sole purpose of making Halo games. Heck they're named after the character 343 Guilty Spark from Bungie's original Halo trilogy. They absolutely will be the ones to make future mainline Halo titles - it's why they exist as a studio.
Halo 6 has to be 343's last crack at the series, so I think the real question is which studio will MS choose for Halo 7. Imagine what ID Software, 4A or Platinum Games could do with it.
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343 was literally formed for the sole purpose of making Halo games. Heck they're named after the character 343 Guilty Spark from Bungie's original Halo trilogy. They absolutely will be the ones to make future mainline Halo titles - it's why they exist as a studio.