Because, over time, it might not be a cash cow as other stores like Play Store improve and innovate. Your post reads like, why should Microsoft invest in IE after v6? No short term benefit? As if. 5 years is still short term and if Apple can't learn from Microsoft's mistakes then it deserves to lose the market share and eventually the profit.
To Apple's credit, 90% of iPhone users won't switch to Android no matter what. So Samsung and Google can innovate as much as they like, apple's user base is not going anywhere.
