The warrior adds abilities the champion doesn't have like danger sense and battle readiness, and loses action surge and combat style. They are pretty similar to a champion, for sure, lol. It's base fighter abilities that are swapped around instead of the subclass.
The expert isn't a combat weakened rogue. They have better skill options than any standard class with more proficiencies, more expertise, jack of most trades and reliable talent. They give up sneak attack but add extra attack so it's actually a pretty decent stand alone alternative to rogues or non-magical bards with things like inspiring help.
Even the spell caster is beneficial over some classes. Adding magical recovery, potent cantrips, empowered spells, focused casting, and signature spells gives benefits over many spell casters.
I find it funny that the "sidekick" expert is a much better skill monkey than any class and the spell caster is a better spell caster than bards.
You are right, though; a person can play an entire campaign with these 3 classes easily and avoid subclasses. I'd play that game.
