Apologies, no need to be rude about it.
Lets look at some numbers.
Damage dealt by thornmail including magic resistance:
( AD * 0.30 ) * (100 / [ 100 + MR ] )
Amount of lifesteal heal:
[ AD * ( 100 / 100 + Armor ) ] * Lifesteal
So, lets say we have a Cait. Level 18, LW, BT and an Ie. (plus AS and boots that don't come into the calc)
She deals 411AD with 35% armor pen and 42 MR (and base 1884 health).
Now, say we have full AD Tank rammus, with Thornmail, Sunfire, Frozen Heart, Randuins and Tabi (And hey, Warmogs cause we're dealing in super unkillable hyperbole worstcase supertank here)
Armor: 435
Health: 3776
So, first, we'll apply the LW, bringing the armor down to 282.75
Damage dealt by thornmail including magic resistance:
( 411 * 0.30 ) * (100 / ( 100 + 42 ) ) = 86.83
Without any bonus health, it's just the extra 10 from Gromp.
So, 96.83 per shot.
Amount of lifesteal heal:
( 411 * ( 100 / (100 + 282.75) ) ) * .2 = 21.48
So the ADC is taking 75.35 damage per shot and will die in 25 shots.
Damage dealt to the tank:
411 * (100 / (100 + 282.75)) = 107.3807969
Total Auto's to kill the Tank = 35.29
So you are correct, when a tank prioritises their spend on countering the ADC, they will indeed kill the ADC without laying a finger on them.
Makes a good argument to not run full AD comps as the same tank will get one shot by Support Annie. :D