とは限らない vs わけではない

not always the case; not necessarily true

とは限らない denies an expected generalization and means something is not always the case or not necessarily true. It is often used with words like いつも, 必ずしも, or 全て to push back against an assumption. It follows the plain form of verbs and i-adjectives, and nouns and na-adjectives with or without だ, as in 幸せだとは限らない or 幸せとは限らない. The だ is commonly dropped in conversation, and the form sounds neutral rather than formal.

1. 全すべての動物どうぶつが野生やせいであるとは限かぎらない。

   Not all animals are wild.

2. いつも自分の思い通りになるとは限らないよ。

   You cannot always have your own way.

It is not the case that; not always so

わけではない denies a conclusion or weakens a claim to mean it is not the case that something holds. It is often used to correct a misunderstanding or to show that something is not always true, as in 高い店がおいしいわけではない. It follows the plain form, such as する, しない, した, i-adjectives like 高い, na-adjectives with な like 静かな, and nouns with である or だ, as in 学生であるわけではない. The tone is explanatory and neutral, and care is needed to keep the negation on わけ rather than making the predicate itself fully negative.

1. そういう類たぐいのものは、どこにでもあるわけではない。

   That kind of thing can't be found just anywhere.

2. しかしこれは昔から常にそうであったわけではない。

   However, this has not always been true.