While some roles can be done, in different ways, by multiple classes, every class is still unique due to the class-specific skills and abilities that are, IMO, enough to keep classes unique. That's at the class level, not the subclass level. Subclass has less uniqueness, IMO, in some cases. Fighter for example, battle master has way more flexibility and uniqueness than the champion due to all the combat abilities they get. No other class can mimic a battle master enough to be considered an alternative for that 'role'.