Are you also comparing D&D3e to Everyway? And how much influence Ars Magica had on Everway? Or the MtG expansion Portal?
People did things before they did other things, that doesn't necessarily mean they do the same thing over and over again. The bring experience, expertise, and a philosophy. And D&D did need a change of philosophy if it was to survive in the new century. Sure TSR imploded due to business errors, but D&D was essentially the same beast since 1974, a whole generation later, people have different views. In the same way that a generation later (after 3e), people have again different views. I would say that 3e was a pretty drastic change, and WotC tried to change drastically again 8 years later with 4e (going for a more WoW-esque game), which was too much too soon for most.
Ars Magica is it's own thing, I look more at WoD (and the other WW titles) for AM influences, which isn't weird as the other part of the people that designed AM went on to WW and design Vampire...