I'm not sure how sensible it is to compare us to The Invincibles. I remember at the time thinking "this is as good as it gets" and realising that almost certainly I'd never see an Arsenal side that good again.
The other thing that happened of course is the billionaires sicking their beaks in, inflating the market and hoovering up a lot of the best talent. And other sides getting wise to our training method and scouting in other countries, we're not fitter than other teams any more and we're not able to cherry pick foreign talent before it's on other team's radar any more.