This is actually why I think exclusives are and have always been a good thing.
I think the souls games have done at least moderately well for themselves overall though. Remember, it was the rising popularity of Demon's Soul's import to what brought it to the states in the first place. I just knew the series was going to be something special when I first seen it. I imported the first joint as soon as it was available to do so.
Also, i wouldn't say they're all the same, as they all had lil nuances that differentiated each other, but yeah, the sole/soul
concepts of the game still apply across the board.
Bloodborne is the most different of them all though and definitely has most of the best and builds on that even more.