As with everything. When the competitor finally gets their socks on and rolls out something decent. It happened with PCs. It happened with Phones. It will happen with Tablets. The fact that competitor companies such as Google and Microsoft licence their software out to many hardware vendors will alone see to higher sales. However the fact that other companies tend to to be cheaper and will have more usability for them who want to seriously mess around with the software on their Tablet will also contribute to sales.
Basically, why spend £400 on a iPad just for facebook, music, video and web browsing when you can spend £100 on a cheap Dell tablet (whatever it will be called when it comes).