getting him makes A LOT of sense to me. You extend him 1-2 years to lower his cap hit. He's 29 and a freak athlete. He likely has 2 elite seasons left in him.
He gives us arguably a top 10 player in the league to pair with Allen. THAT is downright scary for defenses to deal with.
His presence on the field will force teams to bring their safeties down. This alone makes the entire offense more effective.
Give him 18-20 carries a game. Still a pass first team, but more balanced, more unpredictable. This also keeps him fresh for December and January.
Think about how good the play action / RPO game would be with Allen and Henry.