I find this an odd question, simply because the assertion made is patently not true, but also because in some ways, American Idol is one of the worst ways to makea career as a popular musician.
The main reason for that is that the American Idol concept looms so large that it is extraordinarily difficult to establish an identity beyond its bounds. The only contestant to have done so to any extent is Kelly Clarkson. Clay Aiken, Carrie Underwood, Ruben Studdard etc. are all so inextricably linked to the program which gave them their fame that they are finding it near impossible to establish an identity (i.e. brand) outside the confines of “Reality TV star.” Their medium may be music, but in terms of the way they are seen in public, they are perceived in a manner more similar to Colby Donaldson than Beyonce Knowles. It’s little surprise that the one artist who has established a musical identity beyond American Idol, Kelly Clarkson is the one most likely to have any sort of lasting career success.
It’s not the fault of the artists who are on AI that they’re are almost certainly destined for 15 minute flash-in-the-pan careers. That is how Idol works. The show is the star, not the artists it produces. Even the best possible scenario for an artist is fraught with difficulties; Clarkson’s future is still hardly assured and she only managed to get to the position she is in now by releasing a second album superior in every way to her first and having a number of very well written singles driving it.
I can think of a number of routes an aspiring artist should direct their energies toward, and although each is fraught with its own difficulties and requires a lot of luck, remember, getting to any sort of significant stage on Idol is hardly a cakewalk.
1. Become A Producer - People like Kanye West, Pharrell Williams, Lil’ Jon and Dr. Dre produced other people’s records long before they became significant stars. Missy Elliott, too, did a lot of production while she was establishing herself as an artist. This probably works because doing the behind-the-scenes work behind the boards is a good way to establish yourself in the industry - the public may not recognize you yet, but all the big record company guys know who you are - and it ensures you know the way a song works - and even if you’re just singing someone else’s track, you’re still going to do that better if you understand how music works. And the payoff can be ridiculous. Kanye West released his debut album in 2004. By mid 2005, he was on the cover of Time magazine. It took U2 about 6 years, I believe, to reach that milestone.
2. Learn How To Rap - Rappers outweight American Idol artists on the charts by a huge amount. Sure, there’s a lot of people out there, and it’s tough to get attention, but the same is true of Idol. And you could be setting yourself up for a much longer career.
3. Start A Band - Don’t underestimate the garage band route. The Killers, Modest Mouse, Green Day, Maroon 5, etc. have all found susbtantial success the same way other bands have for decades - writing songs, playing them and building an audience. The Arctic Monkeys released the UK’s fastest ever selling album by doing just that (This point comes with a subclause - 3.1. Get a MySpace page). It should also be pointed out that Gwen Stefani is one of the biggest pop stars in the country, and her fame is due to about 15-20 years as the frontwoman of No Doubt.
4. Hook Up With Disney - This could be one of the most promising ideas on this list. (The only qualification is that the younger you are, the better your chances are.) Idol has nothing on Disney; Disney for a start has produced bona fide stars with lasting careers. Britney Spears is still an A-list celebrity; she released her first single in 1998, but got her start as a Mousketeer. Other Mousketeers that found significant success include JC Chasez (of *NSync), Justin Timberlake (first of *NSync and then as a solo artist) and Christina Aguilera. Hilary Duff began on the Disney Channel’s Lizzy McGuire Show, she’s released two albums as a pop star, and while these were successful, her challenge now is to work out how to grow out of her tween audience. She’s made some tentative steps toward adulthood with some singles written by the guy from Good Charlotte, but whether she succeeds or not, her long term career prospects are already better than most Idol contestants. Lindsey Lohan has released two somewhat shaky albums so far, but again, I would say she has a better chance at success than most Idol contestants.
The other advantage with Disney is that, unlike Idol, the institution doesn’t overshadow the star. All the Disney artists have brands beyond being Disney artists. In fact, their Disney connection is hardly remarked upon.
There you go - in answer to your question, if a young person was looking for fame as a singer, there are many routes which could bring them success, but they might have a better opportunity with the Mouse than the Fox.
Of course the once-a-year trickle of AI winners kind of makes that irrelevant.