Great discussion... just going to jump in on one point. Song of defense applies to any type of damage. So if you're in a situation you know shield isn't going to be enough (crits or really high stat attackers), looking at a damage type outside the five absorb elements protects against (say poison or psychic), Song of Defense can be your savior. As far as burning spell slots for it, I think Song of Defense comes into it's own at high level, for life saving moments, when mid level spell slots aren't as critical.
Overall, I think you're making good points. Bladesinger isn't necessarily the default best option for frontliner/tank. But I think Bladesinger is capable of a higher DPR than many of these multiclassing options. I appreciate it for it's striker potential... taking some of the utility focus off my wizard, and allowing me to shift into a half utility / half damage role. This is very party dependent... In my party we have a Bard who takes over some of the utility/support role during combat.