The drastic dropoff is a legitimate point to bring up. I’d put him in the HOF, but it’s not quite a slam dunk case. To me, three extra 2 WAR seasons don’t really make someone a Hall of Famer. They’d make his careee numbers better for sure, but don’t add any “greatness”, per se. He’s a guy I’d out in based on just how dominant of a hitter he was at his peak. I’m also more of a large HOF guy, though.
Here’s a detailed look at his career and HOF case for those interested.
Reckoning with Dick Allen (1942–2020)
I think an interesting case is comparing Allen with another controversial HoFer, Ralph Kiner, who got in by one vote in his last year of eligibility. The knock on Kiner was he only played 10 years due to a back injury. But in seven of those years he was absolutely dominant, and a guy you'd pitch around as he was basically the Pirates only offensive weapon most of those years (which led to the famous Branch Rickey quote, "We finished last with you, we can finish last without you.")
During his peak seven years (which were consecutive), he led MLB in HRs in six of those (he also tied for the lead in his rookie season), and averaged 44 HRs, 115 RBI with a .991 OPS and a 160 OPS+. The result of Kiner retiring after 10 years was that his peak WAR was higher than HoFers at his position, but his career WAR and JAWS score were lower. Allen's peak OPS is comparable at .940, although his counting stats are lower (29 HRs and 89 RBI on average). Similar to Kiner, Allen's peak WAR is higher than the average of other HoFers at his position, but his career WAR and JAWS are lower.
Thus, IMHO an argument could be made that if Allen hadn't played those last three years and simply retired due to injury, it might've helped his cause with voters. It worked for Kiner (and Koufax).