Maybe I'm wrong, but if I'm the Rams I am getting Donald and Cooks locked up in August and letting Joyner play out 2018. If he plays very well then fine, tag him again. Sure, he gets paid $24.8M over 2018 and 2019 but they can afford it and you maintain flexibility come 2020 when decisions on Peters and Gurley come due and they look to extend Goff. I am a believer in overpaying short term in some situations in exchange for longer term flexibility.
Joyner is a very good player and I'd love to read they sign him to a 5-year extension tomorrow. But it's also true that, like Trumaine Johnson, they can decide to keep him through his 6th season and then let him go in 2020 when it's unlikely he will improve further as a player, reap a 2021 Compensatory pick, and go draft a FS. Plus maybe you have to choose retaining young players like Kupp and John Johnson longer term over him.
I'm not going to wring my hands about what happens 4 and 5 years from now because so much is in constant flux and motion. Looking beyond three years out (2020) is impossible. And the Rams best scenario for the next three years as a team considering Joyner could simply be keeping him on tags in 2018 and 2019.