Like you, I went to Madinat not long after it first opened and was wowed by everything. When I returned, I was less wowed, even though I stayed, once again, in the Dar Al Masyaf beach villas, with an almost identical suite.
Like you, I thought that service had declined considerably. The original management team included some really accomplished people, but then this seems to be the norm in many places these days - they put in a top notch team to open the places then sink down to base level once the occupancy is high.
I like the spa, though. They didn't change the treatments, just the operator, so it is still as good as before.