But if I was on the LEL steaming down a mountain pass in the pissing rain during the early hours I want reliable instant braking
Or you could just ride to the conditions and limitations of your existing equipment.
This is sort of like saying that candles are just as good as lightbulbs so long as you don't mind having to light them with matches, having to snuff them out, setting your curtains alight and so on. You shouldn't ride faster than your equipment can allow, but that doesn't mean you shouldn't upgrade your equipment to allow you to go faster.
Although you're right about tyres, disc brakes are a massive upgrade in the wet because they provide instant and predictable braking - when you apply caliper brakes in the wet there is a brief period of non-braking before the water clears off the rims, and if you hit a puddle you lose braking for a split-second as the rim gets wet again.
To go back to the car analogy - have you ever driven an old drum-braked Landrover through a stream and then tried to brake whilst the drums are still full of water? If you had two Landies - one with drums and one with discs - the deceleration in the dry would be the same on both cars, limited by the tyres. In the wet, the one with the discs would stop much quicker.
Discs might be a tad heavier, but the disadvantages are vanishingly small on a tourer/randonneur type bike.