3 more AGI+1 Ath and wearing 7KG heavier armor (so your movement speed is the same) is always better for survivability than just spending those points on 7 IF, and you get more wpf as a bonus. The only drawback is the cost, but experienced players have more gold than they'll ever need anyway.
The main problem with agility is actually speed bonus; 18/27 guys hit just as hard as 27/18 guys, especially with thrusts. But the 18/27 is also just as tough, because he can use negative speed bonus and heavier armor to mitigate damage more effectively than the strength build guy.
1. Find a way to make 7 ironflesh more attractive than +3AGI, +1 ath, and +7kg armor
2. Find a way to reduce melee speed bonus without brutally nerfing cav (i.e. if the equation is a*v^n, decrease a and increase n)
3. Give another round of free respecs so everyone can drop their 21/24 builds