Is no one listening? The mainstream market is nowhere near as big as people like to think it is. A good, in depth RPG that doesn't sacrifice content for voice acting would sell. A lot. There may even be a bigger market than the mainstream market.
I'm not sure why you keep confidently advocating that. I've never seen any solid numbers regarding how big the market is (and how would you even go about getting them). However, there would not be a bigger market. Mainstream, by definition, means "majority."
We don't know exactly how large different demographics are, but we can make inferences. The Wii was criticized from the start for pandering to the mainstream, and years later is still derided with words like shovelware. It remains more successful than the 360 or PS3. The "hardcoe" crowd is angry in the first place because
every company strains for those majority sales, which also implies a substantial gap between market sizes.
It's harder to split because people don't just fall neatly into fan groups, but are on a sliding scale of preferences. The casual market, those who don't play games for hours and hours every day, are the majority. Of those on the other side, many are still willing to buy the "softer" titles even if they'd prefer something deeper. It's not just mainstream and hardcoe, but as statistics go, those at the furthest extreme of preferences, who suffer the most for the absence of games they like, are also the smallest minority. Until making games is so cheap and easy that any company can produce any genre without fear of profit loss, the industry will
never make games with them in mind, which is why they complain so loudly. It svcks, but that's capitalism for you. Most people aren't firmly in that smallest minority, and the mainstream itself isn't "responsible" for what comes out, so you end up with a vague blur of people being screwed over through no fault of their own and people benefiting out of luck, with one side sniping at the other for the lack of any better targets.
Not sure why I'm rambling, though. In the end, it doesn't really matter "how big" the mainstream market is. It's the biggest market, and so that's what companies will strain for.