Replacing Arthur now would just feed into the Parra are a shambles narrative. The guy's last result for a season is a top four finish.
If you want a quality replacement, that replacement has to see the club as an attractive option. Security of tenure, and knowing that your position isn't subject to a poor run of form, despite a couple of years of good performance, does not an attractive position make.
Arthur's been screwed by those he replaced, his 2016 performance was excellent given the circumstances, and his 2017 performance backed that up.
He deserves time now to correct the current issues, and if he doesn't get it, we will continue to be a club that is not a desirable career destination.