So can the fighter also learn both the evoker and the abjurer subclass features? What about necromancer and diviner as well? After all, people in the world learn those abilities, so you should be able to acquire them, right? What if you're an elf and you live for 600 years -- could you learn all classes and their subclass abilities? After all, people can learn those abilities, right? So logically you should be able to learn them yourself as well, especially if you have several centuries of time.
Why would you have to replace current abilities? If the logic is that because the abilities can be learned by people in the world, everyone should be able to learn them with enough effort, correct? So why would the wizard have to replace any subclass abilities? Couldn't they just add more and more abilities, abilities which people in the world learn all the time?
Why would they have to settle for partial abilities through feats? Why can't my character learn the full ability and not a weaker version if I spend enough time?