How can you be certain that changing this detail would result in Android failing? Was it really app sideloading and allowing other stores that made Android successful? I think the biggest contributing factor was that it was made by Google (who had the money to both partner with other manufacturers and eventually make their own hardware), in combination with a lack of other options that'd directly compete with iOS (you mentioned Tizen, but it wouldn't come out for another 4 years after Android's release)
It's not about what made Android successful, it's about what made OEMs ship Android rather than something else. OEMs were conned by Google with an open platform that was then shut by secret deals with some of them. That cost the others who weren't in on the secret deals and it cost consumers in missed opportunities. Take away that scam, and perhaps Android never would have got the OEM momentum it did.
