It would've helped for sure, but not night and day at all. It definitely outsells the 61.91 million of NES. But that's a low bar to clear considering Xbox One sold 57.96 million.
In your hypothetical scenario, Xbox One still gets the S and X and Game Pass becomes a thing. And backwards compatibility probably still gets added in 2015 in this scenario like real life.
I don't think launching with backwards compatibility would've been say more significant than a $400 price tag, no Kinect, and none of the controversy with the reversed policies.
Very little chance Xbox One could hit 70 million units.
Lifetime Sales Predictions
Switch: 161 million (was 73 million, then 96 million, then 113 million, then 125 million, then 144 million, then 151 million, then 156 million)
PS5: 122 million (was 105 million, then 115 million) Xbox Series X/S: 38 million (was 60 million, then 67 million, then 57 million. then 48 million. then 40 million)
Switch 2: 120 million (was 116 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima
