User blog comment:Hamengeri/Small rework for Thornmail/@comment-6301771-20140101065236

Thornmail doesn't need to counter sustain. Thornmail already counters sustain. Armor counters Lifesteal. The damage return is calculated before mitigations, and also counters lifesteal. In addition, healing reduction (Grievous Wounds) is ALWAYS 50%. That's just how it works for simplicity's sake. It keeps it impactful, as a mere 10% healing reduction would be negligible, which means it'd only be relevant after they've attacked 5 times which they wouldn't be doing because you've got a Thornmail.

Honestly, I'd say that Thornmail is in more or less a perfect spot for now. It fulfils a well defined and necessary role at a reasonable cost. There's no need to staple Grievous Wounds to it.