Surely you can see that "easily turned off" is relative to a user's experience, and that
ease of disabling is not the only issue. If you ask most users to turn off a security feature so that they can run your software, they're going to think you're doing something shady.
And that's pretty shitty. Now a lot of people will say "it's only $99/year; what's the big deal?"
Well, first off, that's $99/year, forever, or your apps stop working. That's quite a commitment for a small developer.
Secondly, $99 is steep if you're developing freeware. You should not have to pay a hundred bucks a year for the privilege of giving your software away without people thinking that you're trying to steal credit card numbers or something.
Finally, $99 is steep for a lot of young people. I started developing freeware and shareware when I was in high school. If I had had to pay $99/year to make my app presentable, it probably would have been a deal stopper.
Now, you might say "Sure, but you're an established shareware developer and none of these things apply to you." But they still affect every shareware developer. Ever notice that nobody seems to be have been successful with shareware on Windows in many many years? Everything on that platform has been "free trials" and sketchy ad ware. The difference is ecosystem.
For many years, there was a thriving freeware and shareware ecosystem for the Mac, and so Mac users expected to be able to find freeware and shareware solutions to their problems. They expected to be able to find a handful of cheap or free programs that did what they wanted, and they'd be able to try out each one until they found one they liked. They expected this because it was true.
And free/shareware developers expected there to be an audience of Mac users looking for a free/shareware app that did what their app did. And they could be reasonably sure that if they fulfilled a real need, did it well, and kept the latest versions up on software aggregators, they'd be able to reach that audience.
But those three points above, along with the decimation of aggregators, and the introduction of the Mac App Store have broken that ecosystem. And let's be clear: Apple didn't merely sit by while the forest burned. They clear cut the damned thing and built their app store there. That thriving ecosystem had helped Apple sell computers, but it didn't profit them directly. So Apple set out to change users' expectations away from "find some cheap/free software out there written by a small developer" to "check the App Store", all so that they could squeeze 30% out of the process.
I do not think Gatekeeper is really about security. I think it is about turning the Mac App Store into the Only Way to release software. It's just a little squirt of agent orange to make sure that the forest doesn't grow back.