Honestly this is the best way to proceed. No one from other teams is beating down Valterri's door. And F1 pays well plus he's set himself up in quite a few other businesses such as merchandise, coffee, vodka and others.
If he stays at Dauber it's a safe seat, he's beating his teammate and he can look forward to further good pay.
Plus, if Audi comes in strong he may find himself in a title contending car.