Phil Spencer hasn't given Xbox as much as a boost as Peter Moore or even Don Mattrick did in terms of market share vs. the competition.
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Peter Moore: 2003 - 2007HALO 2, Xbox Live launch, Gears, Forza and Xbox 360
Installed Xbox as a household name in gaming, ended GTA exclusivity went toe to toe with PlayStation and won
Don Mattrick: 2007 - 2014Kinect, Xbox Live growth and Xbox One
According to Wikipedia, while Mattrick was head of Xbox:
Xbox 360 installed base grew from 10 million to more than 76 million worldwide
Xbox LIVE membership increased from 6 million to over 48 million
Made Xbox market leader for a good while
Phil Spencer: 2014 - nowXbox One X, GamePass, Xbox Series S|X, xCloud
Marginally increased Xbox LIVE membership from 48 million to 64 million
Bought over 6 new studios couldn't produce a single launch game
Gears, Crackdown and HALO down the tubes, Xbox One (X) a distant second to PlayStation 4 (Pro)
Gave up on Microsoft Store exclusivity and surrendered to Steam
So far Phil Spencer hasn't achieved as much as his predecessors in terms of sales and market share or even quality video games.