They are actually losing 50m plus wages. What you are mentioning is a gamble, Arsenal does not gamble. They didn't get top 4 with Sanchez and aren't guaranteed top 4 next season if he stays.
Have they ever done anything similar before? They haven't. I would be very surprised if Ozil and Ox stay and leave next year on a free. Arsenal would be throwing away 100m+ all 3 included, shocking business really. They must have some kind of plan.
If they are smart (I know they are not as stupid as they seem) they will sell Sanchez, add some money and get Lemar. Then they will either tie Ozil and Ox down to new deals or sell Ozil for around 40m and Ox for around 15-20.
Same situation with Barkley, entering the last year and Everton confirm that he will sign a new deal or be sold, the difference is Everton are upfront and honest in their admission. Those are the only two options as well. Player signs a new deal or is sold.
The only time I remember a top player that would fetch a large transfer fee leaving on a free transfer is Lewandowski and I don't recall the exact reason Dortmund didn't sell him with one year left, they gambled he would stay and it didn't pay off??