To quote an answer regarding muskets vs armor:
"It is critical to note that early on the difference between armour 'of proof' and other armours is its quality, not its thickness. This changes in the 16th and 17th centuries as guns get stronger - so strong that no 2mm sheet of steel will stop a musket ball, regardless of quality. In addition, the metallurgical quality of armour (or at least its carbon content) -declines- through the 16th century, until many 17th century armours are made of wrought iron (which is to say, iron with no carbon)."
What caused this decline? Was it lost knowledge? A lack of materials?
This is the link to what I quoted:https://www.reddit.com/r/AskHistorians/comments/5udybq/comment/ddth6f6/
The answer you have linked indirectly contains the answer, i.e. the advent of the blast furnace made certain types of lower carbon steel affordable in large quantities.
I'm sure u/WARitter is happy to go more into detail if you ask him in a comment. I'm little curious myself if tempering would have worked with thicker armor, or if the switch from tempered high carbon steel in the 15th century to near wrought iron in the 17th century was because technology at the time couldn't quench a 4 mm breastplate properly without making it brittle.