We don't really see much of a reaction, and the reason is as you say -there was a transitional period where plate armour was used to supplement mail, and it was very long - over 150 years, from the earliest plate body armours in the 13th century to the turn of the 14th-15th century. After the transition to 'full' plate armour mail continued to be worn as a supplement to plate armour, sometimes in fairly completely forms like mail shirts under armour (as remained the fashion in Italy through much of the 15th century), so the difference between 'full' plate armour and what came before isn't cut and dried. Indeed, the transition is long enough and the developments are subtle enough that I find it hard to define just when 'full' plate armour was introduced. How do we define it? As the period when mail ceased to be a primary defense and plate replaced it? In this case we could mark the development of full plate armour to the third quarter of the 14th century, where we see limb defense become more complete, torso defense become more shapely (indicating a more structured plate armour with larger plates) and when inventories in England show full mail shirts abandoned in favor of supplemental mail sleeves and skirts. Alternately, we could define full plate armour as an armour that completely encases the wearer in fairly large metal plates, rigidly assembled together and articulated. In this case full plate armour develops in the early 15th century, as the backplate finally becomes more solid and the bascinet and mail aventail is replaced by the armet in Italy and the Great Bascinet in other parts of Europe. As you can tell, these are pretty subtle distinctions, and they arise out of a technology that developed over decades and centuries. So there was no thunderstoke of a first appearance of knights in full plate armour, but a gradual change that built upon existing technologies. Someone armed at Agincourt in a great bascinet and a solid backplate would not be a shocking sight, but simply a knight 'a la mode', dressed in the latest military fashion and using the latest technology. If we put men at arms from 1360, 1376 and 1410 side by side we see differences, but we see continuity as well. Certainly the later 14th and early 15th century was a time of great innovation in armour, but it was building on technology that was already well developed. Unlike Athena, the knight in shining armour did not burst from its progenitor's head full-armed.

If you want to learn more about the history of plate armour here is a general introduction.