Davaar,
Your arguement is flawed. I suspect this is because your basic definitions of Lift is wrong.
Its often worth remembering that the behaviour of aerodynamics will provide us with an overall aerodynamic force on the aircraft.
We call this the Total Reaction. Most text book introduce it on page 3 and then never mention it again.
Lift and Drag are one way of splitting the total reaction into separate components. We do this in order to simplify our models.
But there are times when it is convenient to merge lift and drag back together again and just think about the total reaction.
Gliding is the case in point. With Thrust taken out of the equation, and lift + drag merged into one force, that leave only 2 forces to look at:
Weight (acting down)
Total Reaction.
Your premise that there are no unbalanced forces is spot on.
Your premise that Weight is balanced (solely)by Lift is false.
What is happening is that Weight must be equal to Total reaction.
If you want to see what is happening with lift and drag your definition of them must be sound.
People get so used to the picture on page 1 of their text book showing lift up, weight down, thrust to the left and drag to the right that they get it fixed in their subconscious that:
Lift is defined as 'Up' relative to the ground, and drag is defined as being horizontal.
This is cobblers and confuses no end of people.
Lift and drag are more correctly defined with reference to the aircrafts flight path:
Try this as a model:
Drag is the component of the total reaction that acts backwards along the flight path.
Lift is the component of the total reaction that acts perpendicular to the flight path.
Ergo they are mutually at right angles.
(a picture is worth a thousand words!)
So our glider has its weight opposed by total reaction. To see whats happening with lift and drag, split the total reaction up based on the flight path.
Lets say the glider is pretty poor, and has a glide angle of 10 degrees. Sketch this. The flight path is inclined at 10 degrees to the horizontal. Our drag line is in the same orientation, tilted up 10 degrees relative to the horizontal. The lift line in at right angles (i.e. 10 degrees from the vertical). If you vector add the lift and drag together, you should end up with a right angle triangle with lift, drag, and total reaction on the sides. Lift<Weight, the shortfall is made up by a vertical component of drag.
Try it again with something that has a very poor L/D ratio. Say a House Brick, falling at terminal velocity. In this case the total reaction is made entirely of drag, so its just (weight of house brick) down, opposed by (drag = weight of house brick) up.
With reference to your earlier post, weight (or any component there of) will never cause a pitching moment. Why? Because weight acts through the CofG. Therefore the distance between its line of action and the CofG is always zero. Therefore it never really causes rotation about any axis, although some explanations for some kinds of stability related design features would suggest that it does - remember my earlier post - they are just simple models to give you a rough idea.
Hope that helps.
Maybe we should move this to Tech Log.
CPB