I totally disagree. Homebrewers have hacked 5e to fix a huge number of settings. All it require it for the core game to come with options built in. The game might have 15-ish classes, but with the assumption that only 4-12 will be used in any given game where they fit the milieu. Different magic-using classes would have different magic systems that fit different settings; Sub-classes could be built with different fantasy genres in mind (spellcasting would become a ranger sub-class, for example). Again, not every sub-class would be available in every game.
Middle Earth: Druid; Fighter, Noble, Ranger, Wizard; low-magic module; slow recovery; tradition fantasy spell list module; traditional fantasy races only
Hyboria: Barbarian, Cultist, Fighter, Noble, Ranger, Rogue, Warlock; low-magic module; all human PCs
Vancian Dying Earth: Fighter, Noble, Rogue, Magician (uses a real Vancian magic system); all human PCs
Westros: Barbarian, Cultist, Fighter, Noble, Ranger, Rogue; low-magic module; slow-recovery module; tradition fantasy spell list module; all human PCs
1e Faerun: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Wizard
3e Eberron: Artificer, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Sorcerer, Wizard
Age of Sail Historical Fantasy: Fighter, Noble, Ranger, Rogue; no magic module; slow-recovery module; firearms module; all human PCs
Early 1900s CoC-type setting: Cultist, Fighter, Rogue, Warlock, low magic module; firearms module; slow-recovery module; all human PCs
Tatooine: Fighter, Monk, Noble, Ranger, Rogue; Psionic feats allowed; high tech weapons module; space flight module
I absolutely believe this could be accomplished in two 350-page books by 1) moving all detailed rules explanations to the DMG and 2) cutting the abstract stuff nobody reads from the DMG.