If Ackster didn't learn Last Stand, he would have to learn another method to control or suppress his overflowing vitality. Otherwise, he wouldn't be able to enter the ocean without being attacked by swarms after swarms of aquatic beasts and monsters.
He could grow strong enough that those beasts wouldn't pose a threat to him. Or he could learn another way to control his life force. Regardless of what choice he made, it would take a lot of time.
Such a delay would either force him to rearrange his plans entirely or to choose a method other than the Sea God Manual to train his mana with. But the mana training methods that can be compared to The Hero's Blessed World Manual can be counted on one, maybe two, hands.
It would be even more time-consuming and challenging to acquire such a method when he could just go for the Sea God Manual.
