I'm gonna start off with I have 0 insider information.
But, to me it seams as though the slow death of DarKastle was the perfect storm of quite a few things. Things (IMO) that contributed:
~Low Ridership
~Expensive to maintain
~SEAS Financial Woes
~Prime Location
~Licensing running out (I think I've seen rumors of this out there)
So you got a confluence of things happening all at once. The first three alone would be enough to close an attraction. Now I'll say, 1/2/5 could have been a work around. You get the story a little reworked, update some ride mechanics, update sets, ridership would pick up. But you add in three and not having the money to re-up a licensing fee, and the ride being expensive, you don't have the reason to change the low ridership.
Now I haven't talked about point #4 yet, but DK's location is a good one. Going through Germany means you had to pass by it to get to one of the few indoor areas for bad weather or hot weather days. So that area theoretically should see high foot traffic, so DK is in a prime location. Having a ride with low ridership, and being expensive to keep up, in a location like that; the decision to close and possibly replace is made easier.
I know DK was loved, and I loved it too, but if Project Madrid and Ireland 2019 are other things that the park desperately needs (thrilling flat and a new high thrills coaster) that increases gate numbers, I count that 100% as a positive. They will be investing in things that will increase the gate numbers rather than keeping a fan favorite for a smaller number of people, because investing in attractions like that is whats going to help BGW be able to bring in even better and better attractions.
Especially if it means that DK's plot goes unused for 4 years (2018, 2019 Ireland, 2020 Madrid, 2021, open 2022) and comes back with something new at that point...I would be fine with it.