The same company that makes GH has always made GH. Harmonics was a co developer and when they got bought out they made Rock Band os it's not like some other company has come in and made a new GH.
Medium Length Guitar Hero History Lesson
Guitar Hero was developed (made) by Harmonix, and published (released) by Red Octane. It actually wasn't very popular when it was first released, I bought it on clearance at Target for $40 (yes, $40 for the game and guitar). Red Octane was purchased by Activision, which gave them the rights to Guitar Hero.
The Guitar Hero franchise took off due to Activision's financing. After Guitar Hero 2, Harmonix was purchased by EA. Guitar Hero 80's was rushed out to milk the license, and what would've been Guitar Hero 3 became Rock Band. Harmonix couldn't make Guitar Hero games anymore because they don't own the rights to the name.
Activision owned the rights to Guitar Hero, but didn't have anyone to actually make the game, so they had Neversoft, whose claim to fame is the Tony Hawk games, make Guitar Hero 3.
So, yes, in fact, some new company did come in and make a new Guitar Hero.
It just contributes to the reason I don't like Guitar Hero 3, it was made by a company who didn't know the first thing about making music games. Harmonix did such a good job with Guitar Hero because they have a history of making music games, with Amplitude and Frequency, and so many of their employees are musicians themselves.
I've heard nothing about a falling out between Activision and Harmonix, and can't find any reference of it. It seems like it was simply a business decision. Sometimes, brand names are worth more than the actual product. Kinda like how McDonald's brand name is worth billions, but the food sucks. I know there's a lot of bad blood now, that came out when they tried to make Guitar Hero and Rock Band guitars cross compatible, but that's business.