Just give it time. Just like Android phones have been chipping away at the iPhone's market share, tablets will do the same. No single tablet will outsell iPad outright, not with the huge lead the iPad has established.
Time will not work to the advantage of its competitors, I think, given the massive head start which Apple has. Alternative products will take away some of the market share, sure, but I just can't see them relinquishing that lead. Apple has a first-mover advantage in a product niche which it essentially created for itself, plus there's the halo effect of their brand name on top of that.
Furthermore, the diversity of alternative products will only dilute their respective market share, e.g. let's say three competing Android tablets enter the market at about the same time. They are not just going up against Apple, but each other as well - it's a really tough market to breach.
Just to illustrate quickly, I'm taking a look at
Amazon's top ten bestselling MP3 players, which is a mature market as the iPod was first launched in 2001. Only two products in that list are non-Apple products, in sixth and tenth place respectively (80% market share to Apple); more importantly, the top five bestsellers are Apple.
If you expand that list to the top twenty, you get eight non-Apple products (60% market share) - but four of them are dead last in the 17-20th place. And we look at the top thirty products, ten are non-Apple, so their market share goes up to 67%. That size of market share in any business is amazing, and ten years on, Apple's competitors still have not been able to dislodge the iPod in that space. I suspect they may be facing a similar challenge in the tablet space as well.