The first two Halo games combined have sold 14.3 million units. That puts the Halo franchise within the top 30 best-selling game franchises of all time. Without Halo, Microsoft might as well not have bothered with the Xbox; Halo and Halo 2 sold more units than the next six best selling games on the Xbox combined (if you're wondering, these games are Tom Clancy's Splinter Cell, Fable, GTA: Double Pack, Project Gotham Racing, and Need for Speed: Underground 2). With a Halo movie on the way, and the facts that Halo 2 sold more units than the first and is still the #1 game played on Xbox Live under Bungie's belt, it wouldn't be wrong to say that the Halo franchise is increasing in popularity.
The bottom line is that Bungie and Microsoft would have to be stupid to end their most lucrative franchise ever after #3. We don't claim to have any insider information, we don't dispute Marty's statement and we don't deny the possibility that Bungie will create new franchises, but based on pure business logic we'd expect to see at least another two games based on the Halo universe. From a business perspective, for Bungie to dump the Halo franchise after 3 would be sheer stupidity.
[Source for sales figures, Image from 1001 Fonts]