I still have doubts about whether a TV with integrated AppleTV features is a good idea or not, as it means they need to offer multiple sizes, and given the premium nature of Apple's usual products with big screens, it seems a poor area to try to break into as there are lots of good, affordable HDTVs out there.
The mini set-top box always seemed the better idea, as the one model is suitable for everyone, regardless of the size of their TV. The main issue really is that you need to put that box somewhere, which is where devices like Google's Chromecast are interesting, though I'm not sure if it'd suit Apple as Apple devices really need to be seen.
Unless of course they can make one really tiny so it simply has to be out of the way, using either power over HDMI or USB as required, and then make the Apple branding a bit more prominent on the TV.
I think it's a fair point that TV's aren't replaced very often though, but with an Apple TV having access to a third party app marketplace that wouldn't matter, as Apple would still be making a cut from app-sales, plus those apps can be enriching other iOS and Mac devices, so it's not as if the Apple TV alone needs to make a huge impact as the halo effect is just as important.