If you're a Japanese guy in 1992, and you've got a Mega Drive and a Core Grafx, I just don't see why buying a Mega CD means buying an add-on/expansion that people should classify as practically its own system with its own library while buying a Super CD means making your PC Engine whole and gaining access to its complete library.
It ain't the price - both are a shade under 50,000 yen. Is it that the Mega CD has a couple extra chips and now it can scale stuff? Is it because Sega is advertising the Mega CD as some kind of "next-level" thing and put the games in a separate kind of packaging? Is it because you read an article that said the PCE was designed with a CD system in mind?
None of those things would really make a difference to me on the user end. I would be much more concerned that both options cost much more than their base systems (or a Super Famicom, if I didn't have one yet), and that both options would allow the base systems to play games that they never could on their own. To me, they would seem like add-ons that effectively function as separate consoles, and both have separate libraries.
At the very least, I think you have to say that if the PCE and PCE-CD are the same, then so are the Mega Drive and Mega CD.
By the way, the base PC Engine's first retail price in 1987 was 24,800 yen (from which it did not drop much), and the CD-ROM unit with IFU briefcase was 57,300 yen. The Duo, when it first went on sale in September 1991, was 59,800 yen. The Duo-R in March 1993 was something just under 40,000 yen. The Super System Card 3.0 was 9,800 yen.
How the different system cards fit in is a whole other can of worms.
Secondly, by and large what CD-ROM games offered back then WAS in fact doable on a HuCARD. ROM size and CD audio were the only additions and while these things are terrific, they are usually unrelated to gameplay. If you load up Gate of Thunder and turn the sound down while playing through the 1st level, all of that could be done on a HuCARD. The CD doesn't add color. It doesn't make sprites bigger. It doesn't speed anything up (the opposite...). It's main achievement is being able to hold more stuff than any affordable HuCARD could but a giant ass Dracula X HuCARD is totally possible and only the audio would suffer.
Why didn't they release Gate of Thunder on a Hucard, then? Could it be that maybe the CD music was considered a
critical feature of the game?
Could it be that whether or not Rondo of Blood could work on a giant Hucard doesn't even matter because a Hucard that big had a 0% chance of existing?
Needless to say, I don't buy your reasoning at all. It's completely stuck in the hypothetical. That you could get the first stage of GOT running on a Hucard is little more than an interesting factoid.
An N64 with a ram cartridge in it is still an N64.
And every game that needed a RAM cartridge came with one, so everyone who owned a plain N64 could play it.