I agree with the OP's points that all 3 have valid reasons why they should extend the current cycle (or more accurately, delay the next one.)
Sony and MS are at best just crawling out of the holes they dug for themselves, and need to build up enough $ to support their next box. Could they afford to build a next box even if their current gen turns out to not be profitable over its lifetime? Of course they could. And like Sony is doing, if the last gen (PS2) box is still selling, that can help cover the losses of the next box. But is it good business practice to do that? No way.
I don't see MS getting out of the business. And the desire to spread to the living room themselves was as strong of a motivator as to keep Sony from claiming it and moving into the PC software area.
Finally, as somewhat touched on, but not fully examined, I think it is Nintendo that has the most to GAIN by releasing first with a new box.
Reasons for Nintendo to release a new box first:
1. Has the 'room' to grow. It can go to HD, get better graphics, better controls cheaper than the competition. All it had to do is go slightly better than the then costing $200-250 X360 and PS3. For MS or Sony to do the equivalent improvement over their current machines would put them back into the $500+ sales range again.
2. It's better to be the one to undercut your own sales rather than let some one else do it. Keep the Wii selling like the PS2 is doing now (and the various GameBoys did while the DS was starting to sell), but be the first with the newer version of the 'Wii'.
3. Move its market upscale and re-claim some of the hardcore lost. As more of the population goes to HD, N. will need a box that accomodates that. By getting one that is better than the current gens but still inexpensive, it forces the others to either rush their new box to market (as MS did to Sony this gen), or makes their current boxes look a bit tired (as in Wii vs the PS2).
Torturing the numbers. Hear them scream.
