If historical bards were historians, and gathered tales and recanted them so that human existence had a sense of place and history because of rampant illiteracy. I can see how commoners would be mesmerized by this and think that it was almost magical that they had such deep knowledge about their past. D&D does not have this problem. Knowledge is common, literacy is common, and written materials are common. The niche the bard had in historical precedent is not the niche they have in D&D. The niche created was one where music was the magic and the historian part was set aside and made a minor feature. I think the better way to make the bard is to pull the class back even further and make them just a jack of all trades. Make them the ultimate dabbler, go back to their knowledge roots. In 5e with backgrounds, allow the entertainer background to define the entertainer stuff, and allow other backgrounds to define them. Basically shave off the entertainer bit. Leave the other stuff. In this way you have a class that can be all things to all people. Entertainer can be there for people who want it and non-entertainer can be there for those who want it.
I also, was influenced as a kid with Lloyd Alexander's books. I think he was awesome. However, he had talents for entertaining, I don't think this was his "class". This goes to my design choice and I think fewer classes with broad arrays of options is better than lots of classes with fewer options. Why cant a CHA rogue be a bard, why can't a sorcerer specialize in song magic? And then there are multiclassing... what! That is crazy!
Historical bards were prophets, magicians, healers, historians, genealogists, eulogizers (which originally included a prediction on the successors of the deceased), teachers, advisors, diplomats, lawyers, judges, and more. They were the scholars of the time and could be called on for a wide variety of duties by their patrons.
Like I mentioned earlier, libraries of books don't move and mass media did not exist even after literature became more prominent. The ability to write and read a book didn't get a book written and circulated in any timely fashion and bards added heralds and messengers to major duties. Bards were the newspaper reporters, television, and Internet of the time as well. News travelled with the bard from one town to the next after books were used more.
It's not like lawyers and judges to sit in court reading precedents from books. A lot of those are referenced outside of court or committed to memory while discussion takes place in court. A bard in a similar role would follow customary law for debate, advice, or rulings in similar fashion. The existence of books doesn't actually preclude any need for skilled memory or memorized knowledge; it only eases the need while the skill continues to be useful.
As for entertainers, bards could and would provide entertainment. They had the knowledge and skills, and one of the key attributes of the bard was the ability to evoke emotional responses.
Keeping that in mind, it wasn't the main purpose of learning poetry. Poetry was used as a mnemonic enhancer to help remember information. Stories were used to teach history and teach "the moral of the story" by parable. A bard would recite what people needed to hear and not necessarily what they wanted to hear in order to maintain custom and help advise. Entertainment was an ability but buts were far more than entertainers.
It's easy enough to make that archetype in 5e. It's my favourite and I will take a sage background, expertise in history, persuasion, investigation, and insight to match the concept. There's no need to bake the historian into the class more than proficiencies, background, and bard abilities that exist in expertise plus some relevant spells. D&D also needed to separate druids and bards, which have a lot of overlap as well, and listed the flavour in the circle druids. If proficiency covers that concept for druids then bards are equal by proficiency or better by expertise anyway. This allows players who want to follow a different archetype to do so because the historian becomes a choice instead of a requirement.
Which gets to my next point. I have a good understanding of historical and mythological bards, and of similar concepts from other cultures. I have my favourite archetype and I can produce it easily in 5e. This doesn't make my archetype the default nor does it mean other players need to make the same bard I make.
Bards are one of the most customizable classes in 5e with any background, any skills, and spells that can be selected from any list before looking at feats or multiclassing. If someone wants more musical focus that's fine and if someone wants no significant musical focus that's fine. A valor bard with a thief background who takes spells shared with the druid list is very much like a 1e bard, for example. This is a pretty good system to give the bard each of us wants while no default is forced.
As for your sorcerer and rogue examples, they can do that. A rogue can take the entertainer background and appropriate skills then go arcane trickster then call himself a bard.. A cleric, druid, sorcerer, or barbarian can do the same thing. A person should always look at the concept and build from that using existing tools instead of just looking at a class. I think players should be building a concept they want to play instead of looking at a class and thinking they need to play it a certain way. For a sorcerer it's entirely possible to make a new bloodline to match the concept and take it a step further working with the DM.
I wasn't sure if some of your questions were rhetorical or not so I either agreed and expanded a bit or answered them. One of the two. ;-)
I do disagree on your take of literacy and forcing them to go back to their knowledge roots because I think that takes concepts away from players when we can already make a knowledge bard.
