"you can't grapple multiple dudes to the ground." Not with BJJ, but you can absolutely use wrestling or judo for multiple attackers, even if it would be really hard to pull off generally. Striking with multiple attackers is also not what the movies make us think though.
You can make grappling visually exciting for sure, especially if you still employ other stuff to keep the fights creative. I love the occasional clutch from Robin, or Seoi Nage from Jinbe, or suplex from Franky.
But for Luffy? He's dealing with guys who have been fighting since before he was born, fifteen foot tall men who are all muscle, guys with weapons, literal dragons, and dudes who turn into lava. Grappling isn't really a good idea. Especially since we've seen Haki for striking contact but it has no specifically good use in grappling.
Not true at all. You clearly don't know much. I can post multiple videos. A jiu jutsu great fighter takes on a boxer on the streets easy. Wrestlers? Even easier. Judo as well.
The advantage is based on a lack of advantage for the other side though. Without a significant physical advantage in your way and assuming the other person isn't equipped to handle it, sure.
But if you're say... a man who is blue belt level in BJJ dealing with a brown belt man, you know enough to take his advantage away if he decides to play by the rules.
This isn't the pre Gracie era, you can't just go into fights assuming the other guy doesn't know grappling every time. If you're again a BJJ blue belt and you think that's enough for a fight, you're probably going to end up fighting a guy with basic knowledge of grappling but with less scruples and a few friends willing to stomp you. Martial arts in non martial art environments can get you dead pretty quick. The one thing with striking is that it relies on standing and controlling distance, so you can both get off your hit and escape far more quickly.