Yeah, beating the missions is challenging, but the replay value comes from perfecting your strategy and getting those S-ranks. And then getting the S-rank for your whole campaign. I think the highest I got was an A-ranking for a campaign.
Yeah anyone can get through Advance Wars without much trouble on normal mode. Hard is a bit more challenging though. Then getting S-ranks is even more challenging. But the real challenge comes in the bonus maps. Some of them I think may be literally impossible. I mean, they're PROBABLY beatable, but I sure don't see how.
And of course if you're just so amazing that none of that is a challenge, you can always build your own maps that will kick your butt.