And I agree... Art direction includes music and just as you need a specific art direction for a specific game, you need a specific music to drive the mood for that game.
On the other hand, Kirill's music was "generic", and by that I mean that his music, even created without a specific purpose, may be used to fuel the atmosphere of a new game, as long as it was advanced enough and not just prototyped.
At the end of the day though, Larian will need to find a suitable replacement. So why not start now ? Kirill's pieces can still be inserted here and there, or iterated upon by the new compositor(s).