Well, if a player is going to rely on magic, then lower their physical attributes accordingly over time. Swordfighting maintains/improves strength - lack of it reduces strength (and any other physical attribute)
Make some enemies immune to magic attacks - or even have enemies get stronger if magic is cast at them (ie they heal when some types of magic are cast at them.) This will force the player to balance magic and melee.
Make some enemies immune to magic attacks - or even have enemies get stronger if magic is cast at them (ie they heal when some types of magic are cast at them.) This will force the player to balance magic and melee.