In a new video, TheStreet.com research analyst Michael Comeau has questioned the long-term momentum of Activision's wildly popular Guitar Hero franchise, which has sold more than 14 million units in North America and recently passed the $1 billion mark in North American retail sales in just 26 months.

In the segment titled "How Long Before Guitar Hero Breaks a String?" Comeau notes that Activision is currently at an "inflection point" with the series, particularly with the recently announced Aerosmith game that tracks the career of the legendary band.

"This is where I start questioning... What are they doing? Is this really necessary? Is it a good addition to the series? I think it'll be a real test. If the game sells well, then [their momentum] goes on, but if there's a backlash it'll clearly be negative," he cautioned.

"I can't see them having 40 or 50 good [Aerosmith] songs to put in [the game]," he added.

As for Guitar Hero III itself, Comeau believes that the game will continue to put up solid results but that MTV's Rock Band is starting to make an impact too. "I think the series will have a good year this year, but we could see it start to decline as people start to get tired of it and if they don't keep the quality up. A lot of people didn't like Guitar Hero III as much as Guitar Hero II, and Rock Band really came in and stole a lot of Guitar Hero's thunder," he said. "So I think [Activision is] going to have to work extra hard to keep [their momentum] going in the future."

[UPDATE] An Activision rep contacted GameDaily BIZ today just to stress the fact that the upcoming Aerosmith game is not comprised solely of songs from the popular band and does indeed include songs from other bands as well.