Tying the game's rules to much to the point value becomes too meta-y. If I build a heavy shooting or fighting army, my guns don't shoot better in a higher point game and my troops don't fight better.
But neither do you have a cap on number of hit rolls attempted. You don't roll

6 at the start of the shooting phase to determine how many models may attempt to fire that turn, and if you did then it would be exceptionally foolish for that roll not to scale with points value.
The winds of magic provide exactly such a limit on attempted spells cast, and it is problematic for that limit not to scale with game size. Imagine, again, if shooting was that way. If you could expect more or less exactly the same amount of cannon and mortar fire regardless of how many points you were playing. Likewise, imagine if only units with ranged weapons were allowed saves against shooting attacks, which is what hinging dispel dice on number of wizards selected amounts to.
There are plenty of narrative reasons that could justify it scaling the winds with the game size - larger armies mean a greater emotional surge attracting warp/waagh energy, more death attracting more dark magic, and a greater fate hanging in the balance. Tying dice available to casters seems equally odd narratively, and, from my own experience, would be far worse, mechanically.
I agree totally this new magic system is ridiculous and needs changing.
How much magic can you use this phase? - dice roll...
ok how much of your magic do you want to put into this spell - decide
ok how much of what you want to put into that spell can you? - dice roll
ok now can you successfully cast it? - dice roll...
ok will it be dispelled? - possible dice roll...
way too much randomness and rolling for each spell going on.
This I strongly agree with - not so much that it's too much randomness, but that it's too many rolls. In practice, it's likely to be the full on opposite of random, as players pick one or two cheap strong spells and reliably spam them out four, five, six, seven times per magic phase.
Under this system there are far too many rolls going on per spell attempt, and with the system strongly encouraging spam casting of small spells on few dice while doubling the dice available, there is also likely to be far more spell attempts per magic phase. This is a recipe for boredom and immersion breaking gamey spell spamming.