The typing issue is not an issue because of the ability to add an external keyboard. I'm not sure why you're making a distinction between "computers" and "tablets." Tablets ARE computers, just in a different form factor. The convenience of the form factor simply means that you don't have to lug around a keyboard (as you would with a laptop) when you don't need it most of the time.
Navigation, again, is a function of software. There is no reason a UI cannot be designed to make manipulation of objects as fast or faster with fingers as it is with a mouse. We're not there yet with many apps, but again, you'd have to be pretty short-sighted to think this will not all changes with time. Now, granted, for certain apps it might be the case that a mouse will always be easier but that's not true across all apps.
As you said, tablets are computers. What makes something a tablet and not a computer? The fact that its ultra-portable and has touch interface. If you're going to forgo the touch interface, and carry an accessory to make it productive, it means that its not really being used as a tablet anymore, you've essentially converted it to a laptop. And a laptop with inferior hardware and OS which makes it a less capable laptop to do productive work.
And navigation isn't purely just software. You interface with tablets and laptops in totally different ways. With tablets, you're using your arm to touch and move around on the screen. You can use whatever software you want, but you're going to be moving your arms around to touch the screen itself. Clearly, it's more tiring despite whatever software you end up using. And it will never be as efficient as using a dedicated mouse.
Tablets have their uses. It would be terrible to try and make tablets an all in one device that doesn't work so well as a tablet only to also not work as well as a computer also. Let tablets be tablets and computers be computers.