I suspect the reason for no BL17 replacement is that if the secret really was out (of just how capable the BL17’s are), it would take sales away from the more expensive offerings, just as you are pondering.
But for those for whom the larger offerings like BL28 are not an option (due to size or expense) the only alternative would be the Balance. I’ve not heard this but by all accounts the BL17 are superior.
I did listen to BL18 a while back. They are also quite large and imposing in a room and I found the mid-bass to be quite muddy.
