WinRT won't gain more than 10% marketshare even if you are generous
I know a lot of people are excited about the Surface and what WinRT could bring but the numbers are a lot more harsh.
There's going to be around a 100 million tablets sold this year. Apple is estimated to sell 70 mil and Google about 25 million. The others are duking it out for the last piece of the pie. Even if MSFT sells MORE tablets than Amazon with its Kindle Fire, the Surface will AT BEST sell 7 million. The other MSFT partners are no way going to sell a combined total of more than 3 million tablets and WinRT is going to have a long road ahead.
We already know that iPad sales are accelerating and might get a huge bump with an iPad mini. The Nexus 7 is doing extremely well and is expected to sell 8 million this year which will bump up excitement for all Android tablets.
WinRT tablets will need to not only sell well but they need find a way to generate momentum. That is going to be hard when Dell, HP and Acer are not really interested in helping MSFT with their experiment. Microsoft's biggest partners are reluctant to jump on the WinRT train when there really hasn't been growing interest for Windows in the consumer space.