Full plate would have been used in the 14th-16th century. Even if they were 5’6" on average, that still makes them substantially smaller than modern humans. If you think the physical fitness of these guys, many of whom had vitamin deficiencies, dental problems, STDs, were inbred, etc, were even close to as athletically capable as, say, a modern marine, you are mistaken.
Furthermore, the prowess of knights just doesn’t hold water. There are countless historical examples (including Crecy and Agincourt) where the knights broke ranks to charge, killed/rode over their own troops, and generally failed to maintain discipline. However, the aristocracy, where knights came from, were also the main producers of historical documentation. It’s doubtful that these guys were going to explain how infantry were more important than the armored charge. To them, battles were a matter of going out and spearing some peasants, and maybe getting knocked down and held for ransom for a while by another knight. (A peasant who dared kill a knight would probably be hacked down by his own knights.) It wasn’t until the 100 years war that nation-on-nation total war became the norm, and victory was more important than personal glory.
And why would knights be better fighters than, say, a Roman legionnaire? Those guys weren’t in the military for fun and glory. Their training was a lot more demanding and time consuming than some knight who played in tourneys and fenced. You might claim the difference is the armor, which was well-nigh inpenetrable by weaponry. This is obviously false, since lots of knights were killed by arrows going through the armor, being bludgeoned to death, in tourney accidents, etc. By the standards of simply killing peasants, they were ferocious, but I don’t think that actually translates to being a serious badass.
Finally, Agincourt wasn’t the only battle where knights drowned in mud or water, only the quickest and easiest example. If you’ll read carefully, you will also see that the knights were tired and worn out by walking/charging. The armor might be well spread out, but it’s still an extra 50-60 lbs, plus a big sword, hot, and inflexible. There were some casualties because of being crushed by the mob, but there were also many who fell off their horse or had it shot out from under them, and were unable to get up. The English suffered very few casualties from mud, because they were barefoot (and many weren’t even wearing pants). There certainly was close in fighting , so why weren’t a lot of English troops crushed into the mud by the vastly superior, mobile, and athletic knights?
Earlier, but still valid example:
Art of War in the Western World
This whole fascination with knights being the epitome of melee warfare is just a product of fantasy literature and the self-serving reporting of a bunch of guys who spent most of their time in war killing unarmored and mostly unarmed peasants. Almost as soon as chivalry was abandoned and war returned to the victory at all costs model, knights (and non-professional armies) fell by the wayside. Go pick up a copy of the book I linked (great read) to get a real, historically accurate, idea of how medieval warfare actually was.