Say he hits .300/30/100 next year. He is a free agent after the 2019 season. If he hits .240/30/80 next year? Also a free agent.
Keep in mind we are already going to be saving a spot for Eloy who will be up shortly after the start of the season. Avi can't play CF, so that position is immaterial to him. We are also chasing Bryce Harper. If that fails, we still have some evaluations to do on Delmonico and Palka. Past that, the Gonzalez/Robert/Adolfo group will have another year of experience in the minors to tell us if they could be ready for 2020.
If we REALLY want Avi's services for 2020 and beyond, we can sign him as a free agent. Let someone else deal with all of the risk of 2019. If he really does blossom into a superstar that we want back long term in a year because we have no better options and it is time, offer him a free agent contract next year, the same as we would have if he had played for us in 2019.
We've done the Avi experience for 5 years now. He hasn't earned a long term contract, or even one that is fitting of going to arbitration over. Non-tender him and move on.