What is tripping you up?
If the modulation of the brake sucks than even if the traction at the tire is high the brake can still lock up because the rotor and pads lock up causing the tire to lock up breaking traction because the brakes suck not because there is no grip available. My point is, is there a point where the disc has coefficient that actually make the tire traction moot. If you have a light switch brake either fully on or off the traction at the tire would be moot because your brakes suck. Heavy or light rider they will lock up.
A big part of the equation is the tire dictates how well we can brake but the brakes a have their own coefficient with the pads and disc. In some situations what if the bite point of the brake is lower than that tires release point of traction. In that case a heavier rider will not brake as well as a lighter rider. I think it the other thread it was mentioned that both riders will take the same distance to stop.
If there is more traction available to slow down at the tire, but the brake bites before this you have wasted traction/braking potential at the tire, making the extra traction from increase weight pointless.
You're the one who is being tripped up by the topic, you have the whole cause and effect around the wrong way. The wheel won't lock or skid until the braking force at the pads and rotor has increased to sufficient levels to overcome the grip at the tyre. The level of braking force required to lock up varies depending on the load at the tyre, there is not one magic limit. Basically, unless you have a catastrophic failure of the brake which causes it to seize, it is always the level of grip at the tyre that dictates when a lock up will occur.
Poor modulation just makes it hard to balance your braking force with the amount of grip available at the tyre, in other words it makes it hard not to exceed the force required to lock up. So with good brakes you may be able to smoothly increase the braking force to the point just before it locks, whereas with shit brakes you try and brake the same, smooth way but you get a lock up. No tyre grip is going unused, the shit brakes are just shit brakes and exceed the tyres grip threshold without warning.
If the pads and rotor do have a limit, of any sort, it will be the point at which they are no longer able to overcome the load on the wheel/tyre and won't be able to lock up.
Do this experiment, (it'll be easy, and obvious) lift your back wheel off the ground and spin the wheel, then apply enough pressure to the brake lever to lock the wheel. Repeat this, except this time you will be riding the bike with your weight over the back, the amount of pressure required to lock the wheel will have changed. This will be the same whether you have great brakes or shitty brakes.
The fact you can't understand that, after obviously applying some thought to it going by the length of your somewhat incoherent post, is why I posted the pic.