I agree with you that *if* we were to pick one casting mechanic that would be a default... Vancian would make the most sense from a tradition point of view. But there are a couple issues with that which we now have to deal with...
1) What do you do with the Sorcerer class?
If we are selecting ONE default mechanic that covers EVERY spellcasting class (prior to casting mechanic swap-outs), does the Sorcerer become superfluous? Considering the whole point of the Sorcerer was to get away from Vancian mechanics in the first place?
Let's keep in mind that we are talking on a purely speculative level, because it's quite clear that the direction of 5e is that of giving each class its own default spellcasting mechanic, and (possibly, but they might change if they run out of ideas) also all at least somewhat different from each other.
That said, you are totally right that the first (3e) Sorcerer's purpose was to try a variation from vancian. The flavor difference was not actually apparently enough at that time to think it needed anything else from the Wizard (well, it got something less, i.e. the bonus feats... but I think at that time the designers were actually afraid that spontaneous casting might be too good, hence also the spell level delay).
I have no idea about the 4e Sorcerer, but the 5e draft for the class is already expanding the concept into something beyond the mere spells. We've only seen one example of sorcerous origin, so we can't take it for granted that ALL origins would grant some transformation powers after using a certain amount of willpower points, but this could at least be a starting point: the Sorcerer could be a class that, whatever the spellcasting method chosen (assuming your original suggestion) is such that the more resources are expended (points, slots, favors...), the more the character "transforms" into something else.
2) Are the people who don't like or outwardly HATE Vancian magic going to be willing to buy the game if it ASSUMES as default that Vancian trumps other methods? *Even if* the book says quite clearly "you can exchange the Vancian mechanics to another one as you'd prefer"?
Yes, they should. But the best way to convince them would be if the designers immediately show, in the first published adventures but also in every PC/NPC example from supplements, several characters that are in fact different combinations of class + spellcasting mechanic. If the designers show that
they are going to use the idea, the gamers will get along. If published material always falls down to the default then clearly nobody believes that the different mechanics are on par.
I myself think that's the preferable way to go with some of the "big" game mechanic issues, because you don't alienate a percentage of your audience right off the bat. HEALING is another one.
I totally agree with healing.
For such fundamental campaign "dials" there should be no default, but rather a small set of alternatives (3 to 5) to choose from.
Same with alignment use.
Same with magic items availability.
Same with XP/level advancement rate.
But those are game-defining "settings". Change one of them, and the game changes
for everyone at the table.
YMMV, but IMHO a spellcaster's mechanic changes the game a lot
for that player but it only changes a little bit for everybody else.
In fact I've just been thinking... I hate encounter powers, but the best way for a player to make me accept encounter powers is probably don't even tell me that he has them

Just use them without cheating and without telling me, so that I might think you've just prepared many slots with the same spell, and I'm probably fine.
And even though THEY CAN swap it out... they're instead going to react with "SCREW THIS GAME, I'M NOT EVEN GONNA PLAY IT IF THIS IS THE CHOICES WOTC ARE GOING WITH!"
Honestly, I don't care for such people at all (but I think they are much fewer than we think).