I don't think it will be that big a task following Wenger, it will be nothing like what it was like at Man Utd having to follow Sir Alex Ferguson.
SAF left Man Utd top of the table, with a squad of players that needed big reinvestments. The club had been penny pinching for years in the transfer market, and it was only SAFs brilliance that kept the team challenging. So when he left it was always likely the harsh reality that the squad was aging and lacking in talent would come to light.
Arsenal are in a different position now though, as Wenger has already taken them about as low as they can go. As ****e as they are they are still a lot stronger than the sides below them man for man, and that's why there is still a big gap from 6th to 7th.
So if a new manager comes in, he will have to do plenty of rebuilding, but I don't see a massive dip on your current form happening.
All this loyalty to Wenger from the board over the years does suprise me to be honest, anyone would think Arsenal where a small club that had won nothing prior to Wengers arrival. Let's not forget they won a couple of titles only a few years before his arrival, and where still one of the top sides in England when he arrived. The way people go on, you would think Wenger himself, done all the work in establishing Arsenal as a big club in English football.