Simplistic as it may seem, from all the way up there, the general overview does look that way. I do not understand why you can view "less new last gen software being pushed' as a separate issue. It's one very important driver for next-gen acceptance. Next-gen gets the latest releases AND it looks better. You need those two factors to overcome the status quo of being happy with your current console of choice.
The reason is because the people who are *not buying* XBox software are not necessarily the same people who *are buying* 360 software.
There are four distinct groups when looking at Xbox+360 ownership:
- XBox owners who still buy XBox games
- Xbox owners who primarily buy 360 games
- Xbox owners who have largely stopped buying games on any MS platform
- Non-XBox owners who now own a 360
There are plenty of viable reasons for #3. For example, many people may be entirely happy with the games that they have and are entirely okay with just playing Madden and Halo 2 for awhile. They may have also picked up a PS2 and they're now catching up on PS2 backlog instead. They may have grown up some and no longer have time or motivation to play games at all.
The issue most people are taking is that your point is that the vast majority of the Xbox base (across the generations) fits neatly into category 2. You have, however, provided no evidence to support this. Meanwhile, there is plenty of evidence to suggest that groups 1 and 3 are also major players simply by looking at hardware sales at price points (particularly that market penetration increases enormously at price points that the 360 is currently still a reasonable distance from hitting). Group 4 is kind of an outlier, but given the huge attach rate, there are obviously lot of early adopters (larger than the late bloom market for the XBox), which indicates that it is also substantial.