RPG elements don't make game RPG, even if dozens of reviewers say that Bioshock is a RPG.
M&B don't set any historical time period and it don't have any historical faction or societies, it only has fewer fantastical elements than most fantasy RPGs. It weapons, armors, etc. things are quite accurate representations of historical weapons, armors, etc. things. But even in them there are mixed different time periods. M&B is in my opinion good game but I don't see how it is any more historically accurate than any other fantasy RPG, it only uses more simulate system than what most RPGs, but such don't in my opinion make game more accurate it only changes how game is played. M&B did sold enough to make profit, but it also was much lower budget game than what you typical top selling games are. And it got sequel only because of Paradox was it publisher, because Paradox view of gaming business is to make games they like.
Million pounds are nice sum (but small compared to budgets of tens of millions that your typical top seller games from AAA publishers have), but 35k backers is quite small, as it will mean with optimistic prediction that game will sell from 300k-700k copies if it gets decent reviews (this prediction is from Paradox CEO about his expectations for PoE ), which isn't much on these days standards where multi million selling games can be considered selling poorly. It is like movies from Finnish studios that get 300k watcher in whole time they are theaters, are considered to be success, but Hollywood movies that get 10 million viewers in their opening weekend are often considered as flops.
So there is market for smaller budget stuff (which PoE is excellent example), but that don't mean that most of those smaller budget products will be top sellers, but of course they don't need to be as they had much lower budget than those your typical top seller products.
What constitutes an RPG is contentious at best, I'd call Shogun and Crusader Kings II RPGs, and Steam certainly says Crusader Kings II is.
Though we may argue what exactly what selling 'worth a damn' means, I think I made my point. They exist, and they sell.