Bungie: Had contract to create 5 Halo titles for Microsoft, had a small grudge against MC due to some development/contract issues. Didn't want to renew their contract after Halo 3, which it expired but they still had to make 2 more games (ODST/Reach). Left Microsoft for Activision.
343Industries: Made up of many former Bungie employees who wished to continue working in the Halo universe. Started development on Halo titles from 2009, that being Halo CE:A and Halo 4. Took over Halo Reach from Bungie which added a new game to work on, also minorly dealt with Halo 3, but it's support wasn't maintained as extensively as Halo Reach.
Microsoft: Owner of the Halo franchise, had a 5 game contract with Bungie, new contract with 343i to make Halo games until further notice. Didn't force Bungie into a new contract and let them go, created 343i from ground up hiring a mass of employees from a variety of companies, a lot coming from Bungie. Forced 343 to make deadlines for example Halo 4, which shouldn't have another 4-6 months development in it, but Microsoft wanted money fast.
So in short, you shouldn't expect Bungie to have come back, they didn't want to make Halo anymore. You shouldn't blame 343i for pretty much anything except the appalling "Limited" Edition issues, and instead focus your blame onto Microsoft. 343i did their best to make a fun/entertaining game for the public, Microsoft were the ones that stopped that.
Another thing to note is that Bungie never made a perfect Halo, the closest they got was Halo CE, their first game. Halo 2 was not a complete game in any regard, Halo 3 was very very good, but still lacked content regarding the campaign. ODST never had a proper matchmaking system for Firefight and Halo Reach was hated by so many until the massive TU. They had decades to learn how to make a perfect game, and only achieved it once.
343i have had only half a decade and made a game worth of being called an AAA title, though still are in a learning process to make the best game possible.