I heard Adelle interviewed recently she said something quite profound: "You have your whole life to write your first album, 6 months to write your second."
And I think that's the problem, bands burst into the public eye, seemingly from nowhere when in fact, they've been gigging, fine tuning and honing the sound of their first album for years.
I thought Razorlight were one of these bands, 1st album was a masterpiece, second was pretty bland with one or 2 highlights. However, they seem to have come back to form with the third - perhaps they're just getting better at their trade.
Problem with Oasis isn't really that they peaked early, they've just fallen into the trap of churning out the same old pap every year. They were never creative - good, yes, but hardly ground breaking.
Then you get bands that do it the wrong way round. Pulp's stock in trade was writing brilliant clever songs about being poor and unsuccessful for many years when brilliant, clever songs were unfashionable. Suddenly they became cool and they had a glut of work available for release, re-release etc. They did some brilliant work on soundtracks (Mile End on Trainspotting etc) and dozens of 'collectors edition' releases with amazing tracks that never made it to general release or even albums. Then they ran out of back catalogue and were bloody rich, so couldn't draw on the same creative atmosphere that inspired their good stuff. Shame really.