I am not sure what the real market is then, though. If it's for people that want to re-live their memories, then a system with built-in games (of which plenty exist) makes much more sense, or one with a digital store (so, the VCS, or digital collections like Atari 50). Whereas people that still held on to their cartridges and want to play them seem to be more the crowd that kept up with newer and homebrew games as well?
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.)