A linking verb equates or attributes one thing to another. In the first place a noun (or pronoun) to a noun, and in the second to an adjective.
1. He is the rascal I was talking about.
2. My efforts to reform are pointless.
When the subject of a sentence acts on an object, then a transitive verb is used. In a sense it subordinates where the linking verb equates or draws similarities.
1. He violates too many rules of conduct. (that's why he is a rascal).
2. My self-improvement plan yields negligible results. (that's why my efforts are pointless).
So noun-attribute linkage and subject-object linkage are similar, and sentences of either sort can be constructed to mean similar things.
The point is, when it comes to pronouns, people make statements linking two things, and they use the objective case universally, ignoring the (grammatically correct) distinction between (transitive) verbs that take the objective case and (linking) verbs that link to a "subjective complement" rather than an object, and therefore take the nominative case.
.