Bard, is CHA? I think you made a very valid point that these are learned things, even the flavor text says so, and it is not granted and not inborn... So the Bard should be INT too.
As a side point I can see the granted power classes switching to CHA and the Sorcerer to WIS. This just makes more sense to me. But there are years of precedence, despite it making more sense.
I agree, and I actually think our old DM who heavily house ruled Bards in 3e did exactly that and used Int for spells.
Besides legacy, I suppose the main problem is that the key ability score that governs a class' spellcasting, ends up defining the whole character.
This is a little absurd, because currently such ability score really only determines the spells DC. Should the player want to cast offensive spells, the spell DC is important, but it is less important than previous editions due to bounded accuracy having reduced the range of both spells DCs and saving throws. OTOH, should the player mostly ignore offensive spells (or use only offensive spells that work with attacks or anyway not allow a saving throw), then the score in your spellcasting stat in 5e means
nothing. Still, you can be sure that almost every Wizard player in 5e will put the highest stat in Int, every Cleric player will boost Wis, every Sorcerer player will boost Cha and so on...
That key ability score is going to have a much bigger effect in 5e on things
other than spells... Once your Sorcerer has high Charisma, she will likely be the best at interaction skills and therefore the most likely to take the "face" role in the party, for example.
I actually like a lot the fact that
it doesn't have to be like that in 5e. You
can play an effective low-Charisma Sorcerer, low-Wisdom Cleric, or low-Intelligence Wizard, as long as you take some additional care with your spell selection. For me this is a strength of the system, because it allows me to have a game where not every Sorcerer is a diplomat but some are weak-spirited antisocials, not every Cleric is highly perceptive but some are absent-minded, and not every Wizard is a genius but some might even be stupid guys that just somehow manage to cast their spells right (the latter is the hardest to accept probably... but then I think of one of my old grandma's friends who was as dumb as a cow about understanding the rules of any cards games they played, and yet always managed to make her cards work and win those games at the end... and make everybody else at the table angry

).