Yeah, I'm bummed about this too. Michael Giacchino can do a great takeoff of a John Williams score in his sleep, and that's part of the problem. I was hoping Desplat would depart in some bold ways from the usual Star Wars score. Giacchino will probably deliver a great score, but it's likely to hew much closer to the scores of the saga films than Desplat's would have. Hopefully I'm wrong.
And that's probably one of the only TV shows to have a memorable score in the last 15 or 20 years.
According to The Hollywood Reporter article, the reason for Giacchino coming on is that the reshoots bumped up the schedule to score the movie and Desplat was no longer available. Even if they wanted a more traditional SW movie and score (which is 100% the right move for the first spin-off movie), I'm sure Desplat is capable of that so I see no reason to doubt it's a scheduling conflict.
That was part of what excited me about the choice of Desplat for the first non-saga film: He'd already proven with the Deathly Hallows films that he could maintain continuity with Williams's themes while turning in a starkly different product.
