Riding the horse is one thing having the rider and mount in full plate with heavy weapons is the one thing that druids won't do and most rangers won't do either. If you are looking for a cavalier expert horse riding is only part of the package you expect when talking about a cavalier.
Druids and rangers are understandably REALLY GOOD at getting animals to do what they want when they want. They are however not heavy cavalry like a cavalier is.
Cavalier is not heavy cavalry. Cavalier is horsemen, the class had dozens of archetypes for every type of rider. Now that it has been changed into an archetype it needs to keep that versatility, not try to turn a class into an archetype and then force that archetype to turn back into the class, that simply doesnt make sense. If you want the class to keep its identity, leave it as a class. If you want the class to become an Archetype, it has to adapt the identity of the base class.
No armor heavier than light, no shield, no banner, no tactics, still very much a Cavalier.
There is nothing whatsoever stopping a ranger from wearing heavy armor, and the only thing stopping druids is the (current) lack of heavy armor that isnt made of metal.
All that said, while expert horse riding might not be the only thing that made Pathfinders base cavalier, thats not an excuse for making it so much worse than Druids. Imagine if Clerics didnt get turning till level 10 and someone said "turning isnt the only thing that makes a Cleric", that wouldnt be a valid excuse for putting off a core part of their kit for half their career now is it?
Now imagine if at the same time Clerics lost turning till level 10, Rangers got it right away if they picked favored enemy undead. Would that be ok?