Upward firing weapons were actually used in a number of fighters - originally in WW1 for attacking zeppelins:
The Schräge Musik approach is also 'Keep It Simple' in contrast to the American over-complicate-it approach.
That term is also interesting as they considered jazz music to be 'queer'.
Ultimately though it was fuel that mattered. When the USA joined the war half way through it meant that Allied planes were on 100 octane petrol, meanwhile the Germans were using some glorified coal slurry and the Japanese were using some tree bark extract made by school kids.
Don't forget the Defiant, a WW2 fighter whose only armament consisted of a rearward-facing turret.
That is exactly what Freeman Dyson said of the Lancaster bomber.
Tho' no sooner had the belly turret been removed than the Luftwaffe invented upward-firing guns, they would slip into position directly under, matching speed and heading, then just let rip.