Marty's music is legendary, but his is not the only style I think that works for Halo, nor is he the only one capable of said style.
Stephen Rippy did a fantastic job in the first Halo Wars and thus far has been the closest to capturing Halo's 'feel' imo.
Niel Davidge's score for Halo 4 may have been less memorable overall, but it did a good job of enhancing the game and had a few standout tracks, (Requiem, Green and Blue, 117, etc.).
I do think Jinnouchi's score for Halo 5 was lacking however; it felt too derivative of Halo 3 at times and even the more unique tracks sort of blended together. In addition, the newer compositions tend to rely far too heavily on the strings, couple that with the lack of memorable melodies and you have the makings of an unremarkable score.
I believe Halo Wars 2's soundtrack has a similar problem; though its overall atmosphere is personified better through the score's usage, and the tracks in the game do have more variety, it just lacks anything standout beyond the reworked theme from the first game and the menu music.
Plus, I never see people wanting Michael Salvatori back, I guess 'Never Forget' didn't have the desired affect. Not to understate Marty's genius but he was only half of the reason Halo's soundtracks were so good and even he says he gets too much praise for what were ultimately collaborative projects.
Halo might not be Halo without Marty in some peoples eyes, but it wouldn't be 'Halo' without Michael either, and considering he is still under employ at Bungie, the point is sort of mute anyway.