Part of the problem is that he's an expiring #2 pick who seems to want to get paid and thinks his current team is hampering him. If he's on a middle of the road 2nd deal that isn't going to break a team, and they get 4 years to get past the injury/slow development that will offset the possibility you might have to pay the guy 20mil plus next year to keep him because he has one good season or even worse finally shows up at the end of the season.
This is a great point. Even teams who might want to gamble on Bagley's potential likely wouldn't want to do so if it means he's a restricted FA at the end of the season and they'd risk either losing him for nothing or possibly matching another team's potential overpay offer.
But if Bagley were signed for four more years at a reasonable number and were able to stay healthy and put up improved numbers he is probably viewed as a better trade asset. Heck, it also potentially makes him a player the Kings would be willing to keep around.
The problems are that (1) Bagley isn't likely to sign that sort of deal (2) he hasn't shown the ability to stay healthy yet and (3) so far Marvin has shown only marginal improvements in years 2 & 3 so I don't think it's likely that he makes a leap in year 4.
As I've said, I will continue to root for Bagley as long as he's a King and it's the best case scenario for the Kings if he improves significantly this season, but I largely see the same player now as his rookie season. He's improved his outside shot, and in a small sample size this preseason he looks a bit better defensively, but he's still largely a talented interior scorer and a gifted though not fundamental rebounder who doesn't really impact winning. I don't mean this as derisively as it probably sounds, but he's primarily a garbage scorer who leaks out for easy buckets and looks for his own shot while still having defensive lapses which allows easy baskets and largely negates what he does well on the other end.
The Lakers game may be an anomaly or it may be a sign of things to come. If it's the latter it means that the Kings are focused on making the playoffs and Bagley isn't seen as a key part of that goal and that the team is past the point of sacrificing potential wins to develop their former #2 pick.
If that's the case, a $12.5 million/year extension for Bagley may look like wishful thinking at the end of this season.