I too am beginning to wonder if the days of 600 or more online players at a time has passed. Giving into that premise, I still believe there is room for a bump in the numbers. Re- configuring the game to be more accommodating to smaller numbers and/ or low (almost dead) time zones could be a better bang for the buck.
Luring new potential players to the fold, should merit them some immediate positive feedback. Here achievements could go a long way. Award perks with achievements, current perk model depends on skills that won't be as readily available to the true "newbee".
As far as Steam goes, I say move a much smaller area of AH2 (say Early War) over there after AH3 has been released. Those longtime players that may be unable to play AH3 due to equipment, may still be able to remain active through steam until they are able to upgrade to suitable AH3 friendly components. It may also, be an area where folks that play during low populated time zones may find a larger audience (if what proponents of Steam say is true).
AH3 however, should remain available thru subscription and 2 week trials, and map size / gameplay modified to cater to the < 600 online player new norm. The option to adjust to a larger population should be easier than it has been to retain the numbers getting lost in the soup of large, inactive, or stagnant terrains.