I think many people (including POTUS) are masquerading as if the assassination was motivated by the necessity to eliminate Suleimani.
He was simply used as a card for retaliation and deterrence.
His death being both highly beneficial to the West and highly offensive to Iran, he was the perfect target, but he wasn't assassinated until the need to retaliate arose and if they couldn't get him they would've attacked a different Iranian target.