Frankly, I think having a sim along the lines of expanding a bit on the current situation in the world today would have more appeal than one based on the Cold War at its height, but it would still have to have the eye candy too in order to even have a chance of appealing to a mass market. The reason I say that is because, to many people (especially computer game/sim buyers), the Cold War is ancient history, and the Cold War without fancy graphics is ugly ancient history. Of course people are wrong in thinking that way, but it's a fact all the same that they do. Nevertheless, 'boring' history can be made appealing, one look at the success of the Total War series confirms that, so it's not impossible, but you have to have the budget to go with the desire if you want to produce something like that, and most software companies would regard that as too big a risk.
A complex submarine sim is always going to have a tough time appealing to a mass market though, unless it has the eye candy to go with the complexity, and even then, the complexity is a turn off to many, if it wasn't, we'd see a lot more mainstream movies about modern subs, which is of course why we don't. That's one thing Ubisoft are abundantly aware of as evidenced by SH4. They had a good shot at widening the genre's appeal with SH4, but if you check out the SH3 and SH4 forums on many websites (including this one), you don't have to look far to find someone asking for help on manual targeting or some other aspect of the Ubisoft range of submarine sims. If people have trouble with that, they certainly aren't going to go for manual TMA and four hours of manouevering to verify a contact, which is why the box art for SH4 has a sub blasting away on the surface with planes whizzing about all over the place. So any sub sim that hopes to have mass appeal would probably have to be one where the sub has a gun that you can blast away with.
We all know that playing about with a modern submarine is a rewarding simulation experience that offers a lot of challenges, but try convincing the average game/sim buyer who wants fast-paced action right now at the push of a button and spectacular eye candy to go with it, and you're probably doomed to failure. That's a shame, but read any trendy magazine or web-based software review of SH4, and you will find that they say it is boring, because they know that this is what the average game buyer will think of it.
I don't like that situation any more than any other fan of sub sims does, but you have to admit that it's true.
Chock