There is a sentence: Monkey king uses a magic stick to fight bad people and help the weak.
As you can see, the author used the weak in the third line from the end of the composition above.
My question is: Would it be better if the phrase be replaced with the plural form the weaks for a correspondence with the previous phrase the people?
... to fight bad people and help the weak.
No, "the weak" is grammatically correct.
weak is an adjective, not a noun. It needs a noun to modify; in this construction the noun is people but it turns out you can actually hide the noun and just use the adjective as if it were the noun itself, to mean "everyone who is described by this adjective." This is probably used most famously in Matthew 5:5:
Blessed are the meek, for they shall inherit the Earth.
In English adjectives that do not relate to number are always singular—they do not agree with the count of the noun they modify. We say "the green apple" and "the green apples," "the weak person" and "the weak people."
Monkey king uses a magic stick to fight bad people and help the weak.
the weak is defined in Merriam-Webster as
noun
weak people
the weak and the powerful
weak is a nominalized adjective, as described in Wikipedia
A nominalized adjective is an adjective that has undergone nominalization, and is thus used as a noun. In the rich and the poor, the adjectives rich and poor function as nouns denoting people who are rich and poor respectively.
Many adjectives, though, have undergone conversion so that they can be used regularly as countable nouns; examples include Catholic, Protestant, red (with various meanings), green, etc.
weak has not undergone this conversion as countable nouns.
