How hard would it be for the 360 or PS3 to simply upgrade their hardware if and when they would be able to add say a more powerful GPU or CPU or RAM?   How much would the addition of these things complicate development?  Wouldn't the addition of a more powerful, and especially different CPU complicate things the most, making a different GPU and/or RAM comparatively much easier to deal with?

Ignore this, I misread the question.

Yup, I don't see your average Joe desoldering the Xenos.

Upgrading either console by the manufacturers with a newer GPU wouldn't complicate things for developers that much. RAM either. CPU would be the problem.

Oops I misread your question and I see what you're getting at now... Why didn't Microsoft and Sony simply follow what Nintendo did rather than putting in new processors, gpus, etc. Microsoft went the way they did because they had a falling out with Nvidia. I'm not sure about Sony, I suspect they thought the Cell would put the PS3 far beyond Microsoft's console but didn't anticipate developers having problems and being slow to get up to speed with it.

Well legend, what if Sony just decided to add a new GPU or RAM to the motherboard? You might get some compatability issues, but the real question is whether developers would have to learn how to program for the PS3 all over again.

And the thing with the Cell is that it looks great now, but in 4-5 years its going to be old news and an upgrade might be nice. 

No, they wouldn't need to learn how to program again.

The easiest way would be to add RAM. The RAM in Xbox 360 and PS3 is one of their biggest bottlenecks... specially on the PS3. No compatibility issues at all, but it would be a big "f' you" to all the early buyers.

As long as the basic architecture stays the same like with the PSP upgrade, things would be fine though you have to remember that developers would have to create games to run fine on the older versions so it's really a moot point in most cases.