Blake Snell has pitched 1 complete season in his 5 year big league career. He has averaged 5 innings per start with a 3.29 ERA/3.51FIP. He has had 1 excellent season surrounded by injuries issues and average to above average performance.
Let's not act like 2018 Blake Snell is every year Blake Snell. Greene could absolutely have a better first 5 full seasons than than Snell. 13.0 fWAR (I extrapolated Snell's 2020 to a full season) over the first 5 years isn't crazy, definitely good but Snell has been derailed by his inability to stay on the field. And is certainly in the realm of possibilities for a guy as talented as Greene in an organization like the Reds to surpass that. Also, Greene, by his own reports, threw over 100 innings this season without issues.