I think the average spells are already viable. The ones that aren't viable are the trap spells and this would not make them viable.
For example, nerfing Shield undoubtedly makes the game more balanced, but it does not make Witchbolt a good spell and does not make it likely that anyone will choose Witchbolt for anything other than thematic reasons.
If you make shield less powerful it is conceivable someone could give it up and choose both Cause Fear and Tasha's Laughter when they would normally only choose one of those "good" spells, but those spells are getting a lot of play anyway and that is not likely where they will turn.
Shield is a defensive reaction and if you make shield less powerful, the other spells that would most likely compete for that spot would most likely be defensive reactions. I think a weaker Shield means more Wizards take Silvery Barbs, not more Wizards take Cause Fear and THL. Unlike Shield, SB is not OP and although this would make those casters less powerful and more balanced then if they had RAW Shield; SB is also one of the most immersion-breaking "unfun" at the table spells in the game.