Yeah, I just cannot see paying any RB big money past their 6th season. There are outliers like Gore etc. but the vast, vast majority of RBs are in (often steep) decline by year 7.
With Gurley, the Rams simply should have picked up his 5th year ($9.3M) option for 2019 and sat tight. Considered a short extension of some kind AFTER 2018. Or if he had still been worth it just franchise him for 2020. Either way having any money on the books for him in 2021 or beyond was foolish and thinking with their hearts not their heads.