If the market for this product is people who only want to play the older ROMs on a HDTV then I'm sure the 2600+ is a fine product.
Maybe I'm projecting from myself, but if I'm that kind of enthusiast that held on and wants to play their actual cartridges, I'd expect compatibility with any cartridges I might still buy in the future.
(Then again, both the RetroN and Polymega seem to do well enough, so what do I know, apart from not liking the idea of these systems as half-baked in-between solutions.)
It seems to me that compatibility with any cartridge would require a recreation of the actual VCS hardware. That sounds expensive to me and is probably why Atari haven't gone down this route. But as you say, that leads to a half-baked solution.
The other option is an emulator that can communicate with the cartridge in real time - but that doesn't exist and I doubt Atari have gone to the trouble of making one (we're all assuming they're using Stella at this point, but I don't think that's been confirmed).