The device will still operate only as long as Glowforge's servers stay up. If there are internet outages, or the company goes out of business, the device will no longer function as a laser cutter.
The firmware is a very small piece of the puzzle. The cloud service is being portrayed as doing all of the heavy lifting. That's image processing, CAM, toolpath creation, and motion optimization/lookahead. Once the motion is optimized so that the motion system will move as fast as possible within its acceleration limits, the result is a list of explicit instructions for the motion system and laser. Accelerate at rate A for T seconds. After X steps, pulse the laser at a power level of P.
Thus, the firmware simply processes these instructions, and actuates the motors and lasers. That's not to say the firmware is trivial. But in comparison to the overall codebase, it is a very small chunk of the complete CNC system.
I'm not arguing that Glowforge is under any obligation to open source anything. But this is a fairly small concession that does not address the main concern that most people have voiced, the inability to run the complete system on your local machine.