This is less about defense and more about prone creatures being unable to stand while grappled though. The fighter in question only has an 18 or 19 AC. Yes he is using his reaction for defensive duelist (and yes if he was a caster he would also have the shield spell), but the biggest contributor to his defense is that the target he has knocked prone makes all of its attacks with disadvantage.
If a grappled creature was allowed to stand from prone, this whole scenario would play out very differently. IMHO it is one of the many big oversights of the 5e rules that a grappled creature is not allowed to stand from prone.
Also, while this strategy might be the best one around when used against solo enemies of large size or smaller, it is exponentially worse when used against groups of enemies or when enemies are huge+ sized.
That's why I like D'Artagnan: because even in his worst scenario (cage match), he's still pretty good. Against a Huge enemy or a group of enemies, it would presumably not be a cage match because Huge enemies need more room and so do groups. He'll use a different strategy like kiting them with his extreme mobility, or picking them off from the shadows.
On the other hand, I guess you could have a cage match against a (Gargantuan) Rancor in Jabba's palace or something, though. That's not going to end well for any of the fighters if they can't find the door button. I suppose Brutus and D'Artagnan could always don a shield and go down fighting but it will be messy. Try to avoid those types of corner-case situations, and look for that door button!
P.S. I don't think it's an oversight that grappling prevents Dodging and also prevents you from standing up. I think it's actually one of the more clever and interesting bits of 5E design.