Thats a bit simple isnt it ?
If on the defensive I dont need a division to hold against one of yours. If half a division is enough to hold against one I have a reserve of 15 divisions and the advantage of interior lines.
Sure, but I also only need one division or even one half division to hold against you.
Then maybe the same number of division as a reserve as you (which i can also use as an attack force) and that still leaves me enough troops to construct a situation of initial superiority (until your reserves arrive) in two or three places of my choice, i.e. places that favor my troops over your troops.
Which means at that ratio I have enough to hold against your concentrated attack and go on the offensive elswhere and roll up your flanks.
Yes. But I do to. The point is I can do all you can do, but more since even with reserves and division holding the line I have more divisions to play around with.
I can construct a double bind for you:
Either you hold still and keep your position.
In that case I can attack at a point of my choice, or several points of my choice, draw your reserves in, and when I have drawn them all, for which I need only enough troops to dislodge your initial deployment and force you to plug the position up with forces almost equal to mine*, I start one or two more offensive actions. Only now all your forces are in the line, while mine are not.
Either you juggle divisions from other sectors, risking a breach there, or you get a breach in the newly attacked sectors. Either way, sooner or later, I get a breach somewhere. And a big one, since once it breaks there is nothing behind it to stop my troops.
And that is without my offensive troops being able to move faster into the depth of your positions than your reserves are able to move lateral behind the front, movement which, unlikey in WW1, can be interdicted by fighters and bombers.
*Here another trade-off comes in: the thinner your initial line of defense, the higher the chance any determined offensive effort can dislodge your troops from their defensive positions, which means to contain the breach you must send more troops than you would have needed to hold in place (since the reinforcements do not or at least cannot expect to benefit from the defensive positions).