Clerics and warlocks don't truly have subclasses. They have domains/pacts which while they use the rules for subclasses, don't break those classes up into different kinds of clerics and warlocks. Take wizards. You can have a wizard of a school(all of the schools are one class, similar to clerics and warlocks), bladesinger, order of scribes, etc. Those are all clearly very different in a way that clerics just are not.
Were this 3e, the school wizards would be the wizards. The bladesinger would be either a new class by itself or if it wasn't a full class, a prestige class. Prestige classes didn't start at 1st level. Only full classes did. If you move subclass to 1st level, you are in effect splitting one class into many(except cleric and warlock).
What 5e did was basically hard code prestige classes into the class system at 2nd or 3rd level.