Corinnguard
Hero
Back in 3e, it was the presence of too many 'dead' levels where the character didn't get a new feature or an improvement of an earlier class feature. The 3e Sorcerer, was a good example of this. At 1st level, a 3e sorcerer got Eschew Materials and Summon Familiar in addition to their spellcasting, and that was it. There was no real incentive for someone to play a sorcerer beyond 1st-level if all they got was more spells. Then along came Pathfinder 1st edition, its' solution to getting rid some or all of the dead levels and ensuring that players stayed with a single class was to fill each level with a class feature with something appealing and a 20th-level capstone ability. If that wasn't enough to keep a player in a single class, there were the class archetypes which altered or replace a particular class feature to meet a certain class concept.Why do you think most people never play a single class all the way up but multiclass instead?
5e sort of has the same problem as 3e did. The base class doesn't off you much in terms of interesting and useful features. All of the interesting and useful stuff is locked up in a subclass. If the subclasses were removed, the base class would need to be stocked up with more features IMO. And these features would need to be designed in such a way for the player to meet whatever they had in mind for their initial character concept.