5 WAR/yr for 15 years is 75 WAR. Average HOF 3B = 67.5 WAR.
He's 1/3 of the way there.
I see him everyday. Rock solid consistent, plays every day, makes all the plays, honed his offensive approach. Should be .280/.360/.500 for the next five years. Not quite as athletic as his brother, but he's a hell of a player.
He's had 1 5 WAR season. (on track for another) This year is his best year and the numbers you quoted are at the top. He's essentially at the top of his aging curve and his numbers support it. Yet, you expect that is his career average until he's 33?
Which 3B have a WAR less than 67 and are in the HOF in the modern age of baseball? All I see is Nettles at 68. Seager will at the very least logically have to reach 70 WAR.
There are like 13 3B in the HOF. Chipper and Beltre are locks. Rolen has a decent chance.
Seager is at 21, but lets call it 26 after age 28 season.
To look at some low end 3B in the hall:
Nettles- Had 44 WAR after the age of 28 (played till he was 43). That would put seager at 70 WAR
Moliter- Had 48 WAR after the age of 28 (played till he was 41). That would put Seager at 76
Edgar was a DH
In order for Seager to reach 70 WAR, he will need to either play productively into his low 40s or massively outplay his prime numbers while in his 30s.
Say he averages 4 WAR per year. To reach 70, he's going to have to average 4 over the next 11 years. If he averaged 5, he would have to do it for the next 9 years.
Chipper Jones, one of the best 3B in the history of the game, averaged 4.4 WAR per year from age 29 to age 40.