I don't consider any player entirely untradeable but Tatum is pretty darn close. Sure, he is not perfect, he is not Kevin Durant (although Kevin Durant was not the same Kevin Durant when he was 23) but you only "decide" to trade a player if you think it is going to make the team better. Even as he approaches UFA status.
I don't see any of the trades mentioned as making us better. I am not sure if this is a mediocre team or not. We are 12-10, that is below my expectation and probably most everyone's expectation but does anyone really think Tatum is the problem? That trading him would somehow make us better? And if 12-10 is an underachievement, what are we if we achieve or slightly overachieve? Are we 14-8, 15-7? Is that still mediocre? Is anything less than a title mediocre?
If we did somehow get to the point where we were trying to blow it up or whatever, I would be targeting players like Towns, a skilled big. Look what Philly is trying to get for Simmons. Tatum is clearly better than Simmons. Most of the packages presented wouldn't even get Simmons, much less Tatum.