Arakune's backdash hasn't always been that good.
In CT it was more of a gamble because of the recovery.
It was really amazing in CS1, but that's it.
CS2 and CSE balanced it out somewhat, but it is easy to punish by throwing a 2a in to test the waters, then autopiloting a punish. You don't even have to really see it coming. In order for Arakune to effectively zone, he needs space to do so, making it hard against rush down types and zoning types, so there's more than just turtling.
I'm sure you're exaggerating with the 10k statement, since he can only get 10k with full curse meter and 2c/j.c, which is situational. On average, Arakune scores about 5/6k with curse, maybe more with resets.
Take the Arakune vs Litchi match up for example. Without a safe approach via staff, Litchi plays a fullscreen zoning game with staff launches, and Arakune has to work around it. Zoning doesn't work too well vs passive opponents.
Arakune has to take into consideration so many things when evading, attacks, pressuring and even backdashing.
He's far from braindead.
Even with the curse advantage he has to worry about enemy placement and bug control.
The main reasons he is dumb is primarily his damage output and mix up, and primer breaking potential during curse, thus it is highly likely that he can steal rounds and matches. Pre-curse Arakune is a push over and he takes a lot of risk to curse the good players. If he was as braindead as you're saying he'd be winning everything regardless of the player.