The problem with your argument is that smartphones and tablets are sold as general purpose computers. They are sold as platforms. The tasks they perform are open ended by design. You can't just take that back.
None of the security and simplicity arguments hold any water because no one asks platform/device vendors to provide support to someone who decided to replace the OS or guarantee their security.
Lets look at that for a moment, your first claim is, if I understand it correctly, that someone selling a "platform" is bilaterally equivalent to "general purpose computer."
I'd claim that people selling a platform have established the set of things such a platform can support. So a Ruby 'platform' only supports Ruby programs, a 'Facebook' platform only supports Facebook Applications, an Android 'platform' only supports Android applications. In contrast a general purpose computer can host any platform, even multiple platforms simultaneously.
Smartphones and tablets are sold as a way of running any program that can run within the constraints of that platform.
General purpose computers are sold able to run any program you can think up.
I'd claim that 'General purpose computer' is a superset of the concept of 'Platform' and that nothing in the secure boot scenario prevents platform conformant applications from running on a platform (no 'taking it back') It does however give additional tools of platform limit enforcement.
That sounds pretty contrived to me, even tautological. Technically these tablet computers are as general purpose as it gets. That's why it takes extra effort on the part of vendors to limit what you can "naturally" do with them. But we're talking about what they are sold as. So here's an example:
"[...] there's no limit to what iPad can do for your business."
That's what platform means. A basis for doing anything without a limit, not just some pre-specified set of tasks. They are expressly different than feature phones or kitchen appliances or automobiles.
None of the security and simplicity arguments hold any water because no one asks platform/device vendors to provide support to someone who decided to replace the OS or guarantee their security.