Sadly I think the rule for successful cast DOES indeed work like former rule 'broken concentration'. The language to me seems identical. But having said that...we can't let it be. The interpretation for a 'dispelled' spell ending all subsequent casting of THAT spell is the way to go. Otherwise these games are doomed to spell casting monotony. Even counting 'dispelled spells' as being failure, players with 3 level 2 wizards have 21 spells to choose from.

I think the proponents for the other interpretation just have not been victimized yet by their playing partners. Once they see the true horror of spammed spirit leech , fire ball, and tons of other really cheap spells they will think otherwise.
As to Sacrificial dagger here is my argument from another forum.
Dagger rules:
1) Once per casting attempt.
2) After casting dice are rolled (no mention of a limit)
3) Sacrifice and gain a power die on a 4+ that MUST be rolled and added to the casting result.
All those rules seem to override the basic rule book. Otherwise they would have said may be used if you have not exceeded the six dice cap.
Even the basic rule book under
CHOOSE NUMBER OF POWER DICE talks about how many you may select from your 'pool'. The 6 dice magic limit refers to dice from your pool. Not the extra die granted by the dagger.
So here a DE Sorceress with the dagger could one dice spells from any of the basic schools of magic or dark magic and if she rolls a 1 or 2, just kill a trooper until she can throw another dice. Note the dagger can be used per casting. This extra die doesn't even come from the original pool so can facilitate the spam of spell casting even more so. So a sorceress with the dagger and 14 power dice (the average) can throw 14 fireballs guaranteed! You really want to face that?

How do you dispel 14 fireballs?