It's only been 3 years since the game's release and we've already got PQ2, P5 Dancing, P5 Scramble, P5 Royal. 3 spin offs featuring the P5 cast and an enhanced edition in 3 years. Persona 4 was also milked to death, but it took Atlus way longer - 4 years had passed before Atlus created their first P4 spin off (P4A)
I wouldn't be against P5 Arena, but I'd much rather have Atlus focus on Persona 6 which supposedly is in development, or SMT5