Here's the thing about that. I know you're right. I know you're being honest. I know plenty of 360 console owners are rebuyers. I'm not undermining you here. But:
The 360 has a great tie ratio. A really great tie ratio. Better than the PS3 or the Wii. Tie ratio is the number of pieces of software sold for each piece of hardware, and the 360's are top shelf both month-to-month and lifetime. This means that if a higher percentage of 360 buyers are repeat owners, we'd either expect the tie ratio to be lower (because someone who buys a second 360 doesn't also rebuy their whole library) or we'd say that it is artificially low, and the 360's actual active owner : software tie ratio is enormous, dwarving everything else ever made by a significant amount.
It seems likely that 360 owners buy a lot of software and unlikely that that they buy a totally mindboggling amount of software, so we have to assume that the repeat buyers are about normal.
One of the reasons here is that most people who rebuy a console trade their old one or sell it and then the person that buys that buys software. So most of your repeat buyers were also driving used sales and thus driving the install base, even if you didn't know they were.
Now, I'm not equating the 360's failure rate to any other console. In fact, let's pretend we're not talking about the 360 at all. Let's talk about the PS2. And now let's pretend no one's PS2 ever broke. Ever. Right? Still, a lot of those people replaced their fat PS2s with slim PS2s (or bought a second one and kept both), right? Sounds fair.
But in spite of this, we don't think of the PS2 as not "earning" all its sales, or of having a lower "actual" ownership number than the sales represent or any other claim like that. We basically accept that whatever the percentage of people who own two or break one and buy a new one or are otherwise rebuyers or upgrade or look for new colours or buy holiday bundles for collectors sake or whatever, it's not something that we actually worry about. We count the sales as honest and act as though the install base is the install base.
Hope this explains the conventional wisdom on rebuying.