Albeit a reasonable approach, given the game itself is then the focus why not at that point make it for a console that already supports those extra features?
I mean one would not develop a new game for the 7800 with the intent of needing HiDef support, right?
Audio being such a sore spot I guess I'm more open to "cheap" improvements in that area (hence the MSU-1 kind of suggestion, or any automatic mod-player, WAV-player or what have you).
I can totally understand, and I 100% agree that the 7800 does need some boost in the audio department, whether that's through POKEY, HOKEY, or some other solution entirely.
As for the reason why you might develop a game for the 7800, given the 7800 doesn't have the features you might desire? The reasons are actually myriad. Chief among them, as a hobbyist developer, might simply to be to see if it can be done. Recall that in the early days of computing, that "can-do" spirit led to a number of innovations that furthered the computing industry, namely by producing hardware that supplemented the computers that were already present. In that case, the base computer was a good start, but perhaps what was needed was a simple enhancement or two. It would be silly to scrap the entire system and simply design a whole new system from the ground up that offered little more than a few simple improvements.
Of course, if you were talking about developing a game from the usual publisher's standpoint of today, developing for the Atari 7800, and indeed developing for any console older than the current generation of consoles, would be ludicrously silly. But for those of us that are in this hobby, that's not the case. It's impressive to see a game come out on a platform with features that would have made our jaws slam thunderously to the floor back in the late 80s. I think for developers working on systems like the 7800 and other retro platforms, it's just the thrill of working with that hardware, trying to see what can possibly be squeezed out of the hardware, and then how improvements could be made that further extended the hardware capabilities of the system.
A prime example of this practice in action is the NES. Consider that the base system was actually quite limited, and it was the introduction of memory management controllers in the system's cartridges that really made the system capable of doing so very much more than it would've done straight out of the box. However, it was often the case that a game's design was laid out first, then a search for existing MMCs was done to see if the game could be matched up to spec with something already in existence. If an appropriate MMC couldn't be found, then that prompted the development of a brand new MMC with all the necessary enhancements, with an eye toward future-proofing by adding features that other developers might find a good use for. At least, that seems to be the usual development cycle for a large majority of the titles that came out for the NES.