I disagree with you on one of your points. Games that run on win98 and windows xp never really ran better on Windows xp. I think that over time people installed better hardware and it made it seem that it ran better under windows xp.
For instance I know, because I've done it, if I install windows 98 on my current PC and run a game I will get better fps than I would on my current pc with windows xp installed. I'm assuming this due to the fact that xp has a bigger footprint on my RAM.