The engineer is relevant to engineering objectives, they don’t need to be any more relevant at attack or defense than any other class. The Soldier on the other hand should be able to lay out some serious damage, the current opinion seems to be that this must be through direct weapons and higher resilience. What I’m saying is also make them relevant using 3rd party items such as turrets and mines.
Turrets are as valuable as you make them, even attackers need to impede the enemy or create a bridge head. This sounds more like you making a situation fit your argument again. But let’s not stray off topic here, the role of the Engineer is another subject. The point is to make the soldier more relevant it should be give more offensives and defensive capabilities, a soldier with a turret is going to give people hassle but at least the soldier isn’t dropping a turret and then completing an objective (effectively doing two roles).