I see no value in killing either Wash or Book. Book in particular was particularly galling as it served no plot development or dramatic purpose whatsoever, for such a backstory-rich character to die that cheaply is akin to James Kirk's ignominious and equally pointless death under some falling scaffolding.
Wash... well, it might have elicited the odd gasp and tear in the theatre but again I don't think there would have been any lessening of drama has it not occurred and it did slam a door shut for no obvious gain.
I rewatch the series every now and again, I don't view the film overly fondly. Maybe I just prefer a more shapely Kaylee