By every report ETH tried several methods to handle this in-house. He gave him 3 months away last season, when our squad was stretched thin during the busiest time of the season with matches coming every 3-4 days, just to get his mind and body right. He also reportedly started giving him a schedule which was an hour earlier than the rest of the squad which had initial success, before Sancho apparently started falling back into his usual pattern of tardiness. None of this has been refuted with any specificity by Sancho, just some cryptic statements about being a scapegoat and double standards, which seem quite obviously untrue when we've seen Rashford and Garnacho be punished for failing to meet the standard in these areas.
You are taking this comment and assuming ETH hasn't attempted a number of different methods behind the scenes. You also claim you're not siding with Sancho while viewing it in the most favorable way towards him, and most negative way towards ETH while also equivocating this with Mourinho and his head-to-heads with Shaw, Pogba, Martial, which were all much more nasty in the actual substance of his words than anything ETH has said. This is not to even get into how much more leeway ETH has initially given Sancho to get himself fit and firing than Mourinho ever offered those players, and in Pogba's case specifically, somebody who was routinely fit and decisive for him on the pitch.
At some point, if all other methods have been exhausted, then maybe you try a public rebuke to elicit a reaction. In this case the rebuke was relatively mild, even if it did not paint Sancho in the greatest light. Instead of responding in any way that could remotely be considered positive, or even apologize to the manager when afforded the opportunity Sancho has thrown a strop and shown exactly who he is.
Also, it's laughable to think this incident has killed his re-sale value more than his disinterested, unimpressive performances over 2 plus seasons at United. The reason his re-sale value is garbage is due to his own poor performance levels in addition to the ridiculous wages he's on which he has come nowhere near justifying, not because ETH and him are having a public dispute.
Finally, if we're worried about squad harmony, reporting has suggested teammates have been left unimpressed with his levels in training also. They also would be aware of the issues ongoing between the player and the manager. Is it better for the squad to leave Sancho out altogether for the time being, or to bring back a player who isn't providing the subsequent level in training, let alone matches, or bring him back into the mix? I don't think it's as clear cut either way, because even if he was back in training there would be a lot of media focus and scrutiny on him, his relationship with ETH, and ultimately how he's used on match day. Removing him from the equation makes it very straightforward for the manager when asked, and gives the squad clarity on the situation. That seems better to me in this situation.