I agree, and responsive to that and other posts. The issue with BS is not simply that they can spike their AC, but they can do so efficiently, most encounters, while retaining full caster levels as a wizard.
Every single 'spike' costs resources that should be used (and would be better off being used) doing Wizard stuff.
Bladesinger 5 has 4/3/2 slots. Presuming even a 4 encounter adventuring day (50 percent LESS than the recomeded average), but still cleaving to the DMG median for XP per Adventuring day (and factoring in Arcane Recovery) he can be Blur-ed for 4 encounters (Arcane recovery and all 2nd level slots gone), have Mage armor running (1st level slot) Shield away 4 times (or only once per encounter) and have precisely enough juice left over to spam a single Fireball.
His Evoker buddy who just hangs back and blasts and lets the Meat shields do their jobs, instead spends a 1st level slot on Mage armor, and drops 4 scorching rays/ hold persons (1 per encounter), 2 Fireballs (excluding other PCs from the damage) or 1 every other encounter, and 3 Magic missiles to strafe any survivors.
I know which Wizard I'd rather be adventuring with.