Imagine lifting a 1 kg object with, respectively, a piece of string and an elastic band. In both cases you will need exactly 1 kg of force to support the 1 kg load, but with the elastic band you will have to raise your hand farther before the object will lift.
The issue with bike brakes is that if the braking system has a lot of elasticity, the brake lever will hit the handlebar before you ever manage to apply as much force as your hand can manage, i.e. before you ever lift that 1kg mass off the ground.
Recall Hooke's law: F=kX (force applied = spring constant * extension)
Two equal springs (i.e. cables) in series have
half the spring constant.
X
max is the X that will cause the lever to hit the bars. F
max = kX
maxA spring twice as long will have k' = k/2. F
max' = k'X
maxIf our spring (cable) is twice as long, the maximum force that can be applied through that cable without maxing the lever travel is halved.
In reality, F
max won't actually be halved, because as you say, the remaining elasticity in the braking system, which is provided by e.g. the calipers, remains constant.
Friction in the brake cables will also have an effect, even once you've applied the brakes and are holding them static in the 'on' position, because friction at the other end of the brake cable will require you to apply force to stretch the cable between you and the 'centre of friction' before you can move any wire beyond it. Even once you've applied a frictionate brake, you'll have to apply force to keep the cable taut to stop it slipping back - even if it wouldn't slip back immediately, but along a slow hysteresis curve.
The hysteresis effect will itself make it harder to control a brake with a longer and/or more frictionate cable: if the brake doesn't give a direct linear pull response to finger pressure, you won't be confident in how to use it under high load, and will stick to braking gingerly.