It's not the effectiveness of dispel scrolls. It's not that Power Dice generated is equal to the total level. It's not the Miscast table. It's not the difficulty of miscasting.

The core magic phase rules have already been toned down. In 6th, all dice are added into a common pool. All wizards can use the dice. Level 1 & 2 Wizards are just dice batteries to power up the Wizard with the bigger spell(s). In 7th, each Wizard has his own dice. Freedom of casting was less.

In 6th, many (rulebook) spells did not require line of sight. The Lores of Heavens and Life are excellent examples. In 7th, these lores were nerfed. Spells like Master of Wood, Master of Stone, Forked Lightning and Uranon's Thunderbolt now require line of sight.

In 6th, there were no hits for the miscast table 7-9. In 7th, there are. The Miscast Table was made worse.

We didn't need so many scrolls in 6th. 5DD + 2 scrolls were considered sufficient. Even with the Slann. So what changed?

It's the presence of wizards that are not restricted to the character allowance and the availability of additional power dice. Horrors, Power Vortex, Blue Scribes, MotBA, and Engines. These units and abilities forced a low magic army to at least consider taking extra magic defense (3-4 scrolls is now common. I didn't use 3-4 scrolls then, I do now). This increase in extra magic defense in turn overwhelmed the medium magic. Now medium magic is not considered useful at all.

The core magic rules are stable, it's just the individual armies. The fix is in the armies, not in the magic rules.