Sure - I don't disagree with you that the problem is easily fixed by forcing players into suboptimal builds via bundling of statistics.
Is that necessarily the path that we want for our game? Not really.
As for the itemization for pure defense is fine part - I actually think that's the reason why pure control tanks are actually something we can support - even if people would prefer to replace them with damaging and controlling tanks every chance that they can get. XD
If I could play Tau and just build my kroot 100% stally defense then that'd be perfect. But I can't, because then my opponents would need to powercreep themselves to defeat my min/maxed out army.
Right there is the issue. If I can min max then you have to powercreep to meet my min max, then there needs to be counters to min maxing, but the counters need to be so powerful that... bla bla it ends up with LW+QSS ranged carries.
Also, on topic to the thread, thornmail sucks. If it had an active that reduced my damage taken by 50% and reflected that damge for 4 seconds that would be ideal. Tons of counterplay in that I just built 2200 gold in 0 damage and you can ignore me to ignore the effect. The fact its passive is "always on" means it has to be weak enough that thornmail doesn't autowin lane, weak enough that you can still focus me, and weak enough that nobody bothers building it.
Also thornmail is another 2k gold defensive item. You can't just fill your inventory with mid game defensive items no matter how hard riot tries to make that a reality.