comparing John Williams to eg., Trent Reznor is a clear case of apples v. oranges. Williams won't be used to score a David Lynch film because it isn't appropriate aesthetically. Reznor won't be picked to emulate Bruckner or somebody in a film where that's thought to work the best, same problem. It's a truly stupid argument.
Classical composers are arrogant and delusional to continue to foster this belief that they are by definition superior musicians just owing to that lifestyle choice. Classical musicians are too often seriously deficient as per any participation in any music outside a really narrow box, too many cannot improvise, couldn't swing if their life depended on it, have rhythmic deficiencies... paltry understanding of vocabulary owing to, their lifestyle choice, which is to remain in some gloried past. For a composer that likes some contribution in a work, and a synergy, a strictly-from-classical player isn't bringing so much to the table to be able to wear that chip on her shoulder and not look like a martinet.
The whole idea of western european concert music being so reified as it is, is deficient in understanding and self-deluding, & the arguments disrespecting electronic orchestrations as per se inferior to the grand symphonic approach belong in a past age anyway. Wake up and smell the coffee, people are exposed to a wide world of music now, and the vehement proponents of this extremely narrow aesthetic point of view are hanging onto these precious ideas desperately, and so often it's nothing more than reactionary behavior. These ideas have had their day in the annals of hegemony.
John Williams is an arranger, and not a lot more than IMO. You might as well use the composers he's aping and hire a music editor in many cases, in my estimation. Furthermore, can he do any sound design at all? I've heard some H Zimmer I found didn't work for me at all, in terms of my own musical values in an orchestration, but I also noticed his work in that Batman picture, and he did some things that Williams will have been HOPELESSLY insufficient for. Horses for courses. You can't diss some chops you lack an understanding of just owing to your own personal tastes, it isn't really better than a culinary preference owing to your particular cultural upbringing.