You are overthinking this. The reason that people are frustrated with him is that he is playing below average defense and providing below average offense while making a significant percentage of the team's total salary, and yet the team seems prepared to let this happen for not one, but two more years. He is, at this point, pretty much what we could expect from Bryce Brentz over a full season based on MLEs, but costs $20 million dollars more.
Understand that I have loved Hanley for a long time, and was pleased with this signing when it happened. But he has delivered exactly 1 fWAR over three years at this point, and that means that the bar for replacing him, at DH no less, is so low that it's criminal to think that the team would simply let that option vest out of a lack of imagination. Eat some amount of the remaining contract for nothing in 2018 if you absolutely have to, but OP is right that it would be bonkers for him to still be on the team in 2019.
All that said, this conversation is a little premature. We don't necessarily have a better option today, so he is playing out this season either way. Then, at some point close to the deadline next year, if he has remained healthy and is on pace to hit the PA threshold, you try to trade him and eat his salary. If not, you give his job to the best hitting kid in AAA, and DFA him. Or, best case scenario, he somehow learns how to hit again next year and rakes, and we are suddenly happy to have him!