Please tell us what the HMD, apart from cuing and visual target tracking, is meant for (just like the HUD)? Is it situational awareness (again like the HUD)? If so, then please explain where exactly the critical info which is displayed on the HUD and the HMD will be displayed on the canopy? Since, apart from cuing and visual target tracking, one major use of an HMD is that no matter where you look all the critical information for the operation of your aircraft is presented right in front of you,increasing situational awareness by leaps and bounds (Hence why HMDs are still invaluable without HOBS missiles). Let us not forget that there were helmet mounted cuing systems before there were HMDs and they worked fine for their purpose (and would with your hypothetical canopy display as well). So, in case of this critical situational information being displayed on the canopy itself, please explain where exactly would this info be displayed for it to always be in the pilot's view? Would this extensive display system just be a glorified visual target tracking system?
Then there still remain the nitty gritty issues such as the increase in the aircraft's weight which is always far more critical vis a vis the helmet's weight (so much so that manufacturers have been opting for 2-piece canopies instead of a single piece as on the F-16), your cockpit acting like a beacon for the enemy at night, etc.
The solution to the helmet's weight problem, believe it or not, is to build it with lighter materials and further miniaturisation of its components, a field in which mankind has made tremendous progress and, given the pace at which this field of tech is progressing ala google glass, will continue to do so.