In the end, its quite complicated when you want to copy paste the exact structure of the Japanese English into language so "Samurais" would be just fine since you are taking the Japanese word and using it in the English context adding the "S".
It's possible to do it that way, and people would understand what you were trying to say, but it would be incorrect. Just like if you were to say "I eated a hamburger yesterday." would be comprehensible, but grammatically incorrect. English is a language that has irregular words and rules governing them, and many of those rules are more concerned with the sound of the word rather than the spelling due to how much spelling has varied over the centuries. As a result Samurai sounds better than Samurais, thus it is more in accordance with the established rules.
One way of getting around this problem is to use words like Samurai and Zaibatzu as descriptors which is why one might often see "Samurai warriors and Zaibatzu conglomerates" when referring to them in the plural.