Valve relied on old glory as well, Half-Life 2, TF2, Counter-Strike, DOTA, but new contenders in the form of Blizzard start popping up and challenging them.
Same thing will happen here, even if it isn't Iron Harvest, one day a game will just pop up and push CoH2's status as defacto the current WW2 RTS game off and plant itself on the top and Relic will keep scratching their heads wondering why and how it happened when the simple answer all along was that maybe listening to your community and not being so money hungry would have helped, a lot.
Money is the reason why Relic is in business otherwise they would be bankrupt. I think Relic is smart in concentrating all of its effort in AOE due to larger audience. COH is a great franchise but the amount of investment $$ it takes to produce this product and with already dwindling customers, I dont think its a right time for them to release the game just yet. I think what would help Relic the MOST is take a page from Blizzard and make COH2 a free to play game and monetize cosmetics as the community grows.
The biggest problem I see for the future of RTS game is that majority of players are much older compare to average gamer. I rarely see young gamer play COH2 or any RTS.