I did notice that, and I wasn't particularly surprised since you and I seem to agree a lot. I think the main objection to your standard-core-spells idea will be that it cuts down on choice for those who don't necessarily want those spells. I might suggest 2 or 3 core spellbooks that mages could choose from to add a little more variety in case someone doesn't want the blocks or they'd rather pay for decoys than seeking dispel or something.
We do think so similar! Because I thought the same as well. But then I ended up with what i came up with.
(Your "elegant interesting solution" that I said I would think about provoked my thought processes on this.)
My thought process developing this idea:
In custom card games, there are various models.
Some, like Game of Thrones, allow for a single "Restricted" card that you may have legal copies of.
Some, like Netrunner, encourage a certain size deck and limit you to c.15 influence (out of school).
Well, you can immediately see the 66 cards to fit 120 points comes from Netrunner.
Because Netrunner melds opportunity cost (takes up limited space) and points buy (our spell points).
I then translated Restricted as "what if every mage had 12 spell levels of Arcane for free?"
But I felt this gave too much flexibility in the end, too much unrestricted budget inflation.
In the end, the reality is 2 Dispel + 2 Dissolve + 2 Teleport is a compulsory minimum buy.
And it was this ability to buy essentials on the cheap which gives Wizard his power base.
So I felt give them to all. Wizards gains the least and is nerfed because 66 spell slots only.
Part of the fun of this "Pro" rule is you KNOW I have 2 Nullify, 2 Block and 2 Seeking Dispel.
Believe me because I love playing tricksy builds, this gives you amazing bluffing opportunity.
You don't need Decoy (that Retaliate could be first thought as Nullify then Block) to bluff.
If you do want Decoy (and you know how much a fan I am of its style), it's a Novice spell.
Then I thought about the expanding card pool collection, ever growing competition for inclusion.
The game is actually nerfing itself in its diversity so here was a chance to also expand diversity.
As well as weaken equipment (enchantments are great anyway, you always get a 1 use benefit).
I know MW has a lot of "y = mx + c" linear relationships hidden with it (all good games are maths).
spell's lifetime benefit = (average lifetime x per round benefit) - constants (spell points, action, mana)
Actually that's the simplest relationship model but we'll go with it
Everyone was talking about altering the multipliers but it's the spell point tax that hurts
Anyway, that's the reasoning why I decided against a flexible Arcane budget
I just felt that KNOWING these spells exist in the opponent's book adds so much awesome mind games.
And of course it stuffs the Wizard (because Wizard may think it's a Nullify and Fireball to find it's a Block).
Anyway, I'm not against the idea of more free flexibility.
But thematically, every Mage apprenticing with the same base spells also feels better.