Alt-coins don't necessarily compete with Bitcoin, but rather strengthen cryptocurrency in general. I doubt Bitcoin will ever completely be replaced (where it disappears), not without some fundamental flaw discovered an alternative can solve or some superior feature Bitcoin can't emulate.
The internet is NEVER about being the best... it's about:
(1) Being easy to use.
(2) Though imperfect...
Connecting with humans on some "lowest common denominator" level.
(3) Rapidly building a critical mass.
That's why BitTorrent crushed Napster...
YouTube crushed MySpace...
And strange, simple Twitter just freaking exploded.
Ripple is incomprehensible...
So it's not a threat to Bitcoin...
And the alt-coins don't have the "critical mass" BTC does.
In the above context, Bitcoin looks good.
http://falkvinge.net/2011/05/11/with-the-napster-of-banking-round-the-corner-bring-out-your-popcorn/