However, the main reason these apps gained all this traction is that the Web is simply not made for small devices with touchscreens. It's the discomfort of using web browsers on mobile that created the tipping incentive to adopt these streamlined apps. As long as people keep using more comfortable devices (eg. laptops) that can easily browse the Web, this segment will not have that "discomfort" incentive to move away from the Web and onto more restricted platforms.
Yes, there could be other incentives - you mention junk sites and security issues - but these aren't strong enough to outweigh the main advantage of browsers, which is the breadth and momentum of the Web, something that users still value. I hope I'm right.