Changed his name to sound more French? The horror! Of course by that standard we'd have to judge Frédéric François Chopin (born Fryderyk Franciszek) or even Ludwig van Beethoven (who sometimes styled himself 'Louis'), harshly. Not to mention a certain Endre Friedmann, who became André outside Hungary, but later settled on the more marketable 'Robert Capa'.
A bit hard on Sibelius, too, to write him off as a slacker resting on his laurels. His attempts to complete an 8th Symphony well into what would for most people be their retirement years seem to have been pretty tortured, and not helped by bouts of depression and alcoholism. But the work he did finish will be listened to for as long as there is music. I doubt Mantovani had any such ambitions for his oversweetened arrangements of popular tunes, 'light music' that was never intended to do more than entertain. Less forgivable, perhaps, is the sort of overhyped work sold for inflated prices by people like Peter Lik, the photographic equivalent of Mantovani, but without the accessible pricing.