I'm not sure I agree that the decline of the Tower wasn't well underway right before Hawkwing. The AS began to lose their good reputation with the Trolloc Wars and even had to add a third oath against lying back then...

I don't agree that Cadsuane and Moiraine are examples of AS "living in the world". Moiraine especially spent all her time out of the Tower, but almost exclusively dedicated herself to hunting for Rand. Cadsuane sought adventures. In their way, they are typical AS, just living apart of everybody else while outside the Tower.

I think a greater presence of AS in the world, in the sense of involvement, has pretty much stopped with the Trolloc Wars.

I'm not sure I see what impact the presence of AS had on the decline in population. There never was enough of them to really improve living conditions (not since the TW, and probably not since the AOL, actually), especially that pretty much the only service they will volunteer is occasional Healing. They could do much more "menial jobs" that would count a lot, as Elayne pointed out that she felt guilty Elaida devoted her efforts only to a private garden and when she mentionned this Elaida offered her to pick one farm she could do the same for. There's weather regulation too. AS could do a lot, but they haven't in a long, long time.

A factor I forgot, I think, is the loss of the Ways around Hawkwing's time. That's when the more remote areas with ancient cities lost their fast way of communication/travel. If we look at the map, the nations that fell were mostly cut from water ways (or in the case of Almoth, it's only access east were passes that close during winter and early spring).