こそすれ

Used to say that one thing may well happen, but the opposite or a worse outcome will certainly not. It means roughly "rather X, but certainly not Y" and is always followed by a negative clause.

A written, somewhat literary pattern found in essays, editorials and formal arguments. The second clause is typically a negative expression such as 〜ない, 〜はずがない or 〜筋合いはない, and it contrasts sharply with the first. It is stronger and more rhetorical than plain こそ, which simply adds emphasis.

• こそあれN3「こそあれ」 expresses concession — 'although there is A / it is A, ...' — and its following clause may be affirmative or negative; it mostly attaches to nouns and adjectives, and the variant 「でこそあれ」 also exists. Both patterns are written language, but their meanings differ, so they cannot be interchanged.
• 〜こそ〜がN3「〜こそ〜が」 covers a broader range, and 「こそすれ」 can be seen as a fixed written form within it. In its general form, 「が／けれど」 conveys 'A is indeed A, but B'; the following clause need not be negative, nouns and adjectives can attach as well, and the register is neutral.
• こそN3「こそ」 is a register-neutral particle that attaches to nouns, case particles, the て-form, 「からこそ」, and the like, emphasizing the preceding element as 'precisely ...'; its following clause need not be negative. 「こそすれ」 attaches only to the ます-stem or サ変 nouns and requires a negative following clause, so the two cannot be interchanged.
• はおろかN1「はおろか」 means 'let alone X, even Y ...', with X and Y escalating in the same direction, whereas 「こそすれ」 affirms A and negates the contrasting B — the directions are opposite. 「はおろか」 attaches to nouns or to a verb in its 辞書形 + の, so the two cannot be interchanged.
• からこそN2「からこそ」 emphasizes the reason — 'precisely because ..., that is why ...' — attaching to the plain form or to noun/na-adjective + 「だ」; its register is neutral and the following clause need not be negative. 「こそすれ」, by contrast, means 'only ever A, never B', so the two cannot be interchanged.

The key to distinguishing 「こそすれ」 is that the following clause must be negative, creating a reverse contrast of 'affirming A while negating the contrasting B'; it attaches to the ます-stem or to サ変 nouns (nouns that take する) and is used in written language. 「こそあれ」 is also written language but concessive ('although A, ...') and allows either polarity; 「こそ」「からこそ」「〜こそ〜が」 are register-neutral and never require a negative; and 「はおろか」 is a same-direction progression, the opposite of the reverse contrast of 「こそすれ」.