Turn based tactics isn't that niche and neither is 3rd edition D&D.
Sure, turn based tactics is less popular than the mainstream genres, but it's not 51 people small. Battle Brothers peaked at 8k online and had long periods where it was hitting 2-3k peaks every day, with a home brew system, no magic, 2d sprites with no animation and no legs. And it had a $29.99 price most of the time although there were some sales.
Obviously not every game can expect that level of success, but Pierre chose to make his game overly niche with his design and marketing decisions, like gameplay based on reload spamming, a tiny hideous interface, pogs for graphics and a $44.99 price.
Obviously there's a huge audience for D&D based games as Pozzfinder illustrates.
Of course only a small percentage of those people would buy a more prestigious game, but it's still possible for a developer to tap into some portions of that audience if his game is marketed and priced right. Heck, the Pozzfinder devs actually promoted KOTC2, so it was definitely a possibility that some of those people would buy a well done D&D combat simulator.
But $44.99 + current marketing is outside of impulse buy range for most people who are interested in the genre but not specifically familiar with the Dev. It's a huge leap of faith to shell out that much for a game that *looks* prosperian in every respect.
Saying "this game is limited to a niche audience, therefore we should overpay for it" is whale mentality and it rewards bad behavior, encouraging the Dev to continue sabotaging himself. That's how you end up in the same situation that war gamers are in.
I think you make a few good points and it's true that Pierre could have priced somewhat lower in order to have tried to reach the sweet spot of maximal rewards. I'm sure he tried to. I suppose my main point is that I don't think the sweet spot is as low as some seem to think it is and that even if he priced it at $2, this isn't going to reach some sort of critical mass like Minecraft or Angry Birds or something.
I'm not sure that 3E D&D or hard turn based tactics are as popular as you seem to think. The Pathfinder games had two or three things going for them that KOTC2 doesn't: RTWP that reminded people of the Infinity Engine stuff, pretty pictures, arguably licensing. Yet, people whined about the difficulty, the fact that it was more complicated than the Infinity Engine games, etc. Even games that are more popular like the nu-XCom stuff got there in large part by being relatively easy and streamlined/dumbed down compared to the originals. I think in order to turn this corner, you'd have to look at what people tend to say about the original Fallouts now. (Hint: many of them can't believe that anyone wanted to play them and figure Fallout starts at Fallout 3). Really where I'm going with this is that the exact type of content that appeals to people about the KOTC games (character building, approaching encounters and trying to figure out how to solve them with the abilities and resources the party has) is exactly what you're talking about shaving away in order to make more money.
I won't pretend to speak with experience on exactly what happened with the war game market, however, tastes do change. That stupid Dragon Age tactics minigame they used to promote the release of Origins years ago was ridiculously easy. I sent feedback saying it was a lot of fun, I just wished they made the hardest difficulty actually challenging. Later, Bioware discussed the feedback received and it was overwhelmingly that the game was too hard on normal. So... YMMV, but understand that the vast majority of people, while (perhaps) not being borderline retarded, aren't very good at tactical thinking or attention to details that is important in this sort of game. In order to appeal to a broader market, you start grinding down the difficulty and appeal of the game to its niche. I think it's been pretty clear since the beginning what sort of game Pierre was keen on making.
On top of that, the fact that it's coming with a complete toolset to build your own dream adventure, perhaps with a far more relaxed difficulty and/or some nicer art assets also warrants consideration.
The niche audiences aren't being punished or rewarding bad behaviour. It's a simple fact of business that if you're chasing a niche market segment that has a very small amount of people, you need to charge more and roll the dice that that niche will be underserved enough and want it enough to pay for it. If you want broader appeal, you can design with those considerations and a lower price point. You see this all over the gaming industry. Do note though, that I'm not at all suggesting that there aren't things that Pierre could have done/could do to try and broaden the appeal of the game. I'm just not entirely convinced that changing the price is the highest priority on that list.