So what I think you are saying is that AppleTV is fine at 720 for the reasons you list, when, in (likely) fact Apple could (if it wanted to) roll out a 1080p version of AppleTV for probably not that much more cost.
My arguments are not really about what fits what people have now, or what applies to XX% of households now, etc. I'm mostly focused on the wish that AppleTV could have really locked in on what will probably be a long-term standard (at 1080p) rather than something that would have been a "wow" in about 2003 (when 1080p was not yet available in consumer hardware).
My belief is that it seems unlikely- even with an Apple premium- that a 1080p Apple TV would cost more than about $499 if it had been launched yesterday. And I very strongly believe that if that is true, Apple shouldn't be making the resolution decision for consumers, but at least giving us the option to choose to pay up for 1080p version, vs. taking what might work fine for XX% of people now for less.
Just because you believe that TVs below $2000 aren't 1080p, doesn't make that so (do some searches). And even if we make that statement into the "majority" of TVs, that's only "as is" now. Let's see what happens in just 6-12 months.
In the meantime, "as is" (with a 720p max video spec) creates the possibility of having to buy the movie twice- once now to fit what most people might have now, and again when most people replace their TV with what will probably be a 1080p set. We've already played that game- maybe a few times- to go from VHS to DVD and now maybe BR or HD-DVD. Of course, this is exactly what the Studios want, which might help explain why all of them signed on with this HD rental deal. But the Studios could have still chosen to rent their films only in 720p, aiming to still move most people to potentially have to buy/rent films 2 more times- even if Apple delivered a 1080 AppleTV.
If Apple did roll out a 1080p AppleTV, it would naturally encourage all the buying, ripping, HD camcorder encoding, podcasting to all be done at 1080p now, which would work on both the current and future HD sets in most homes, looking as good as possible- now and in the future.
In short, downscaling from a higher quality source (at 1080p) maxes out the picture quality on a 720p set today, as well as tomorrow's 1080p set. But being satisfied with a 720p signal today because that's what Apple chose to offer as max resolution, just won't look as good on tomorrow's set.
The cost of the hardware to "future proof" this little box should not be so much that we need to go through 2 generations to get there. Instead, Apple could have just gone on and delivered the future yesterday (or even last year), leaving all the other pieces (TV, sources of content, etc.) needing to "catch up" to Apples full potential. Wouldn't that have been more Apple-like?