Im not sure if the 9500 was the first cuz I know I could put a video card in my old PPC 7500, and it came with a decent vid card for doing digital video (which I still use till this day).
I myself have often wonderd why the iMacs and all in one units have omitid things like PCI expandability and I still don't get it. Perhapse it is economy of space or perhapse it's as you say, to encutage people to buy higher end units. From personal experience however I don't think it's ither. I think the all in one iMac's and the like, target a user base of people who just wan't a system that, as the name implies, are all in one. They don't want to worry about graphics cards or souping up their machines, they just want a unit that they can rely on and use almost as an appliance.
Also, iMacs are not as non-upgradable as you think. There are tons of third party manufacturers who have developed CPU upgrades and even video card upgrades for modles as low as the generation A iMacs. Granted, these things are not what their all cracked up to be but neither are any upgrades, which brings me to another point.
Upgrades to some extent are a bit of a fantasy. Sure theoreticaly you can just keep changing the part that gets outdated and the like but in the end bottlnecks on even bigger parts (i.e. motherbord, and system bus speeds) that the smaller parts depend on are going to develope because of the advance of technology. You are basically going to be spending the money on upgrades keeping your machine state of the art as just buying a new comp. I remember back in the day when the PII first came out, I was gonna upgrade my regular Pentium but the speed increes was not that incredible, why? Well the motherboard was an older modle that was not fast enugh to keep up with the new speeds.
As far as the new iMac having an outdated video card, I hardly think that just cuz it's not a GeForce 4 it's out of date. Find my a game that a GeForce 2 can't run and I'll be a monkeys uncle
Seriously, my friend has a GeForce 2 and I have a GeForce 3. We play plenty of hi-end games and both our cards handle the games perfectly. I think the hardware is WAY ahead of it's time on this one and will be for a good while.
Heck, not too long ago, I was forcing my poor little iMac's ATI 128 to run Quake III at medium settings, and while not spectacular, got playable results.
All in all however I agree with you. For those of us who are toataly into hotroding computers and keeping them on the cutting edge, Apple has no real low end expandability. They only really have low end, and High end computers. No middle ground.
None the less, the G4 towers offer plenty of expandability. In the long run, considering the money you WONT be sepending on upgrades, due to the very high system specs., I think they are worth the expense. That's just my opinion.