I don't think a majority of people will even go to alternative storefronts to get the software in big enough numbers to matter. I think it's more of a legal concern than a monetary (sales) one, but I could be wrong.
I think you're right, as it stands now, especially for platforms like Android with one canonical store that already has many free offerings. I use fdroid because i love the philosoply and am willing to put up with it but to be clear, the apps there are unprofessional and ugly and I get why most people don't.
But if this became a common practice I think people absolutely would. Tons of professional quality games instantly available for free is such an incredibly good value.
