You realise these same companies who are producing 7" tablets have/are also producing 10" tablets. You think a 10" andoid/blackberry tablet is going to be any more of a commercial success than a 7" model?
Its likely any tablet of any size is going to pale in comparison (in regards to devices sold) to one developed by apple because of the brand name amongst so many other things. Also IIRC when publishing their financial results, Apple typically catogorises many devices under the same umbrella i.e. the ipod range. Looking at things from their perspective, Apple will sell more ipads with both a 7" and 9.7" models than with just one available model.
I agree with you about the other companies likely failing with a 10" product also. I don't agree with Apple being better having a 7" also. This isn't like laptop, having a come enough size is going to cause confusion.
Finally, as I've said in other threads, people are missing an important factor in devices like this. You can't just scale the UI because the interface is a finger not a mouse pointer. If you keep the same number of pixels but reduce the size, the UI doesn't work anymore because buttons are too small. If you change the number of pixels then it's another form factor for developers to support and existing iPad apps won't work on it. It's just not a viable size from any point of view.