The 9400M isn't even really that good a GPU.
Only certainly early iMacs (the education models) got the GMA950, but yes, the GMA950 was a product of a time when integrated graphics were not as good as they are today.
Sidenote: Until just earlier this year when, not because it was slow, but because it suffered physical damage to its display, a ThinkPad running a Penryn CPU was my main computer for photo organization, etc.
I'm very familiar with computers from about that time, because that was when I was in the throes of shopping for what would become the ocmputer I used for the next 7-8 years (by odd mistake, really) and I can easily see why one of the Sierra-capable MacBooks would be a desirable system. Heck, I've low-key wanted one basically since the Penryn+9400M launched, both before and after the plastic unibody restyle.
About the MacBook Air: I think a very specific part of Apple's market (and perhaps even some new Mac users) were intrigued by a system like that. Parallels have been drawn all over between it and the new MacBook with Retina display. It really only feels slow because of the RAM limit and the slow disk interface. The point wasn't the performance though. The thing we all see with good Core2Duo laptops is the computing requirements plateau in extremely plain action. Software is getting more efficient, and unless you're actively getting new cameras or games all the time, requirements aren't increasing for most people the way they were before.
We'll see what the next big thing to become mainstream, but the biggest things that really drive the need for better performance is the fact that computers from before the late Core2Duo era were usually extremely poorly equipped, even for their time, web sites, and then after that, just physically wearing out.
And that's the reason I'm not rushing to put a new screen in my ThinkPad T400. I can do the repair, I'm confident that it'll work and go back together... as well as a system that's as old as it is and has been taken apart to the bones for service can be. Everything on it is looser, the keyboard is obviously wearing out, fans are starting to spin more slowly or make rattling noises, and for an OS that has been more efficient than OS X for the past decade, like Windows Vista/7/8/8.1/10, the upgrade from 4 to 8 gigs of RAM is a lot less straightforward and obviously necessary.
(Neat sidenote though about at least some of the late geforce-equipped MacBooks, not sure if this is just the 320M or both the 9400M and the 320M versions, they can run 16 gigs of RAM, which is something Intel 4-series notebooks are, as far as I've seen, are unable to do.)
All that said *2, the MacBook was never really meant to have a good GPU specifically. It was meant to be the cheap model, an affordable entry level computer (within the context of the Mac, anyway) that was also a pretty nice machine, but in comparison with the MacBook, the "Pro" had a much better screen, a discrete graphics chip, better default configurations, and higher max configurations.