So, in your example, they balance at 7 rounds of combat. What if the combat ends on round two (coincidentally, after the wizard has unleashed his two spells)? Not so balanced anymore.
Yes, you can theoretically balance mediocre over a long period with awesome over a shorter period, using a bit of math, hps or some quasi-arbitrary quantification of 'usefulness' or 'fun' of awe-inspiring magic vs bland sword-swining. But it's very theoretical, because (1) you can't gaurantee they'll always get measured over that longer period and (2) because, really, no amount of mediocre ever really adds up to awesome, anyway.
1) is the DM's problem. It the DMG tells the DM that the average battle is balanced at X rounds, then the DM knows softer enemies favor the caster and tougher favor the martials.
2) is the player's problem. The PHB tells the fighter's player that if they take the Exploit warrior feat to get the "Earthquake Strike" and "Hurricane of Blades" exploits it'll increase their spike damage and awesomeness at the expense of their basic attack.