Didn't push back on what you think his biggest negative is, because I disagree with it. Almost in the same way as rebounds, turnovers can easily be cut by changing your play for the worse. Rather than try to hit guys making backdoor cuts and lob attempts for easy baskets that are higher risk, you just make safe passes around the perimeter, rarely get your teammates easy baskets but have a low turnover rate. Not looking to have a Sam Bradford as a point guard at all.
Edit: When did a 2.7 to 1 assist to turnover ratio become not just bad, but unacceptable?
Ignoring the rest, this is where I think we have the most fundamental disagreement on who Marcus is as a player and what people who would like more of a real PG are concerned with....
Nobody is asking for Sam Bradford, they're asking to not have Carson Wentz where the risks are immense and the rewards modest.
High risk is great, if it comes with high reward. Marcus has the 3rd highest TO rate of PGs in the NBA this year, the 3 ahead of him....
TJ McConnell who was awful, Caruso, really bad on offense, and Russ... but Russ had a slightly higher turnover rate (0.5%) with a MUCH higher reward rate in assists (12.2%),
If Marcus was getting an assist rate in the 30s you could live with 18% turnovers. Maybe even if he was upper 20s but also using more possessions on efficient offense. He's not though, he's using a low number of possessions on mostly inefficient offense, assisting at a decent but unspectacular rate (basically he's Bradley Beal in AST rate) and turning it over like he's Westbrook or a high usage post player.
As to ratio... it was less about A/TO (where he's lower end of PGs but not terrible) it was about how many turnovers he has for how low his usage is, usually you're fine with higher turnover rates from your highest usage players, because it's understood that anyone who is consistently going to the basket and making plays is going to have a few more turnovers (strips, offensive fouls, uncalled hacks, etc.)
Marcus is a good player, I think we should give him another year at least... but any discussion of him as an offensive player should note that he provides very little positive value on that end... his value is almost entirely in his defense. There are some reasons for concern long term there, and certainly he shouldn't be considered some untouchable core piece of this roster, he's an important piece, but also the most expendible of the guys who has been here a while.
To me the reason not to trade Marcus is that he's a niche piece, not many teams want/need what he brings and those that do aren't going to swap him for the pieces that would upgrade us.
Edit- and who knows, maybe Marcus comes back next year and refines his decision making, starts figuring out when to take risks and when not to and it leads to a reduction in the number of empty or negative possessions he creates. The potential is there, he's not a bad passer, he's a pretty good one, it's just that much like his shooting he often makes really poor decisions, often in high leverage situations.