Thresh passive doesn't 'fall off' and hasn't been for years aka it gives the same amount of armor and AP for every soul (0.75). However it falls off in normal game time's lategame comparatively to the usual carry builds because those have multiplicative stats reaching a huge powerspike at 3/4 items and it takes way too much time to have adc level of damage with a tank build and linear scaling (sthg like 30 hours^^), same for the other infinite scalings. Nasus a bit less, but still and he is easily kited and killed.
That's why infinite linear scaling is actually far from broken. Old devourer wasn't really a problem after it got nerfed the first time. (yet it will bring tank builds because it gives flat damage with no other synergy stat than attack speed). Will be less crazy in dmg output and tank melting though, I suppose.