Is the solution to make it harder? Or is the threat of scammers and the insecurity of the OS used as false flag to make installing software outside of the profitable walled garden much much harder?
I doubt that side-loading impacts revenue all that much. Alternate stores are the real, potential, risk to $.
I think the solution is to come up with a balance between the needs of different groups of users. People here see the phone as a general purpose computer they should be able to modify and use for all kinds of novel tasks. This is great, and should be fully supported.
But there are also many, many more people who see the phone as an important way to enable a higher standard of living. Giving them access to information, government services and banking for the first time. They are not technically sophisticated, and don't need or want a general purpose computer.
So, we need platform providers to come up with ways to work out who is who, and give each side what they need.
