How about this then? If we replace him with someone, we need to be comfortable that they can do a better job.
I don't think Arthur is great coach but he isn't a shit coach. Coaching Parramatta isn't easy and there have been countless factors that have contributed to our demise during his tenure. Those factors are the same ones that have seen us not perform well during the tenure of Hagan, Anderson, Kearney and Stuart as well. If we are relying on a 'bounceback' effect then that is the wrong way to look at things. A change of coach alone arguably won't help in the long term. Also, if we change the other factors (which I have previously mentioned) its highly likely that we will start performing well on the field.
Besides, who is a great coach? Bennett? Bellamy? Who else? Beyond a few elite coaches, I reckon the majority are all of equal ability.