I think you two are being too harsh, maybe just make the effects of playing an illegal formation bigger. Playing a match can only be good for the team, you get skill points and packages, your players get more goals and assists in their stats, for those who monitor these things. Condition can be controlled by a decent manager, the only problem would be injuries, but an injury isn't so bad if it's to one of your reserves, and if the effect is made bigger, I think all managers would play their reserves against a team playing an illegal formation.

I don't see why a team should forfeit a match if they play with an unbalanced formation. In real life football, a match is only forfeited if the side cannot come up with just 10 players, I think. Teams used to play with 2 defenders, and nowadays, teams play with no strikers.