Of course the other point that's usually overlooked about plate armour is that it is a weapon in itself to some extent, if an unarmoured combatant attempted to grapple with a heavily armoured opponent they'd most likely be pummelled to death by gauntlets, kneecops or the elbow thingies. Hell, just imagine getting squarely headbutted in the face by someone wearing a heavy steel helmet.
I could never get my head around the Agincourt melee situation, where lightly armored English men-at-arms and longbowmen were able to defeat heavily armored French knights.
Surely, I thought, even though the French couldn't move, their armor still worked and they could still swing their weapons?
Then I saw this video. Nothing to do with historical battles, its a video from a protest in Germany taking place in a muddy field.
https://www.youtube.com/watch?v=O9l3bLAx4NgThose riot police are wearing equipment that is probably significantly lighter than medieval plate, and yet hundreds of them are rendered completely ineffective by the weight of their gear.
That protestor in the monk robe is simply dancing around them with impunity knocking them off their feet for fun.
Another thing is the definition of 'grappling'. It doesn't have to mean WWE-style suplexes or rolling around in the mud. A simple trip, shove or leg-sweep to knock over an opponent in heavy armor is very effective, and indeed these kind of techniques feature very heavily in a 'battlefield' derived grappling martial art like jiu-jitsu.
After upending an opponent, a skilled grappler has a choice to continue into a wrestle, or simply leave the opponent on the floor and move on or strike for a weak point in their armor, which would definitely be the choice against an armored opponent.
I recently played BG3, and many of these mechanics already exist - shoves, trip/knock prone, disarms, throw, falling damage, joint damage effects (eg one character got a dislocated shoulder) etc. These can already simulate an awful lot of a 'grappler' skillset, even without the full-on 'grab/wrestle' side of things.