というものではない

というものではない rejects a simple rule of thumb: "it's not as if X automatically follows", "X doesn't necessarily mean Y". It follows a plain-form sentence, and the sentence is usually a condition with ば plus the result people take for granted. A noun or な-adjective can take だ before という or go without it.

The speaker does not say the rule is false, only that it is too simple. What really matters often follows in the next sentence.

ではない or でもない

というものでもない makes the same point more softly: "it isn't necessarily so either". ではない is the firmer denial and suits a correction or a warning.

The pattern turns up in essays, editorials and serious discussion. In conversation it often shortens to ってものじゃない.