the extra attack feature is a feature, spellcasters also get features that alter their damage, like twinspell. also martial charachters extra attacks are supposed to be balances for their lack of scaling damage skills and spells with higher damage or utility.
by level 5 wizard has access to scorching ray, haste, etc.
the level 5 wizard has two hastes, which is essentially extra attack for two encounters, if they choose, they can summon shadow blade and throw 2d8 daggers twice a turn. [...]
mages get big damage from spell use and basic damage from cantrips. casting 1 cantrip per turn is not how mages will realistically play except maybe eldritch blast warlocks. whereas attacking twice for a fighter or barbarian is basically peak play at level 5.
That's not how I'd classify things. Cantrip scaling is the direct comparison to Extra Attack - a resource less ability that grows in power with level. If you're bringing in Metamagics like Twinned or consumable spell slots, then you also have to consider Fighter features like BM maneuvers or Echo Knight's echo or even Eldritch Knight's spellcasting.
And no, wizards cannot summon shadow blade and throw 2d8 daggers twice a turn. Aside from the action cost to get all that set up, both spells require concentration. In fact, shadow blade better serves to show my point that crits are roughly balanced: with two 1d8 melee attacks from a 5th level fighter vs one 2d8 shadow blade attack from a wizard, the wizard will get an extra 2d8 on a crit but the fighter has twice the chances to crit.
I agree that wizards have more options and generally more power than martials. But when talking about crits specifically, things are already fairly balanced.
As for casting cantrips, idk about you but in any game I've been in with an appropriate number of combats per day (>3), spellcasters don't have enough spell slots to cast leveled spells every turn, and thus fallback on cantrips decently often. Plus the BA spell rule; if I cast a BA spell then my action-spell use is restricted to cantrips.
