That would only be valid if the Asgard tech would be capable of replicating the ZPM's.
Given the level of technology of the Ancients, the Asgard were close to them in terms of technological development, but not on the same level.
Of course, we don't know exactly just how many advances did they make in the 10 000 years since Atlantis was originally abandoned (if they even did due to their own problems of preserving themselves), so either way, I think it will be possible for Humans to start producing the ZPM's eventually.
They already have the necessary information, the rest is just a matter of making them.
And since at the end of Atlantis series, humans had a wealth of Asgard technologies along with Atlantis itself... I think it won't take too long.
As for why not give Atlantis it's own power source... they did.
It's the ZPM's.

The Ancients to my recollection used them to power virtually everything that required enormous amounts of power... and keep in mind that 3 of them in Atlantis would be able to sustain the shield for LONG periods of time (we're talking about a century - which is no small feat).
And almost any power source with finite amounts would be eventually depleted.
The ZPM's are no different... and let's face it, the Atlantis expedition was able to do plenty with just 1 of those things without having the other 2 most of the time.
I think that Weir's decision to give the other 2 ZPM's to Earth and the kids was a bad move (after the replicators took control of Atlantis and subsequently lost it).
The outpost on Earth could have continued to go by with it's original ZPM as would the EM dampening field with the kids.