I am not using any other crypto currency at the moment, but I don't think any of them will ever overtake bitcoin. Its the standard of all, its like money now, do you see money competing with anything else? I really don't see that. So I feel bitcoin will hold it own in the market and would probably keep growing.
But you do see different currencies competing with each other. A century ago, British pound was the world currency. Now it's the USD. In the future it itself may be ousted by yen, or some other currency. It's true, no altcoin so far could provide decent competition to Bitcoin, but I don't think that the future appearance of such an altcoin is that impossible a scenario.