I'd save most of my income even if my ROI was -10%, because money is more useful in large quantities, especially when you break certain thresholds like:
*Can take 2-3 years off work when it's a good time to do so
*Can mix cash and and labor when joining a company to get a big equity stake
*Can buy tools and gifts without even looking at the cost if there's a solid benefit"
Cash is more useful/leverageable in large numbers than small for most middle class-ish people. Any gains you make are gravy, saving would be smart even if it cost you money to save.