There are a few positions that because of the way the NFL salaries are structured you just can't pay big bucks to. And one of them is RB.
Rams need to 1) get all they can from Gurley while they have him for a reasonable cost; 2) steadily transition the offense to depend on him less; 3) draft a good RB who can step in after him.
There is no way you can have Gurley, Donald and Goff taking up a third of the team's salary cap space. You just can't afford to have enough good players in other positions to be playoff competitive if you do that.