It's an interesting idea. I wouldn't eliminate direct damage entirely (as another poster mentioned, that could have the unintended effect of having mages shooting crossbows).
You could push this avenue of design by simply having spells limited to about the same (or less) damage as a martial character can output in a single round.
The design philosophy regarding spells has traditionally been that because they are a limited resource, they have to be able to do more damage than a fighter can. This has been a general trend over the various editions. That said, damage spells have stayed fairly close to where they originally were, while monster HP has increased significantly. As a result, control spells (which have remained consistently useful throughout the editions) have improved in value relative to their direct damage cousins.
By limiting spells to dealing no more damage than a martial could inflict, the design could push casters towards a control playstyle. Eliminating damage forces them to play as controllers, but may result in undesirable side-effects. For example, what if the fighter falls unconscious and the caster is bereft of options to take advantage of something like burning? Are their only options to heft a crossbow or retreat? IMO, that's not a desirable scenario.