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I agree that having the option to run unsigned code allays many of these concerns for now. It's just easy to get a little concerned when one looks at the trajectory (unsigned code > sandboxing required for App Store apps > APIs only available to App Store apps > signed code require for default operation, etc.)
Here's the difference. I see this as a backtrack of trajectory. They have a perfectly good 'locked-down' system in iOS. If they wanted to continue on that trajectory they're already there! It would be easy to bring that system to the Mac app store.
The fact that they took the time to make up this whole new gatekeeper thing seems, to me, to be a deviation from the trajectory: An acknowledgement that they literally can't monitor the Mac like they have the iPhone.
In fact, I give strong odds that iOS eventually goes to gatekeeper and the IOs store approval process is scrapped. Not this year, but eventually. I feel very good about those odds.
Policing the app store is a thing Apple would prefer to not do. It takes a LOT of time and resources and it hurts their platform in terms up software update speeds. Apple would LOVE a better solution on iOS.
I think Gatekeeper is it and I think putting it on the Mac is their way of testing it out to see if it works well.
Many here don't believe this. I do.