Sooo. What about rogues? Their AC will generally be around the 16 mark.

Even taking armor you're at 16 with chainmail. Vs 16 with mage armor.

When you reach plate. Which may not even be by level 5/6 armour starts to edge ahead.

But unless you're planning on having everyone sword and board and ignoring all character flavour you use the tools your class gives you.

The lesser AC is mitigated by being a caster who is ranged. And has spells like shield for the rare time the enemy slips past your front line.

Dropping a caster level is a big deal when its your primary source of everything you do.

Personally I almost always dip 2 into fighter for armour and action surge. But I'm simply arguing the point it's not necessary.