とは限らない: withholding a definite conclusion
Use とは限らない to allow exceptions to an apparently general conclusion. Distinguish it from a blanket denial and track the possibility left open when it follows a negative clause.
Not true in every case
高い物が長持ちするとは限りません。 means that expensive things do not necessarily last a long time. Some may last well; it does not mean that every expensive item soon breaks.
The entire clause before とは限らない is the judgment that cannot be treated as universally true. Just as price alone does not establish durability, the expression withholds a conclusion automatically drawn from one characteristic.
Following a negative clause
In 明日、雨が降らないとは限りません。, the claim that it will not rain tomorrow is not certain. Rain therefore remains possible, but the sentence does not forecast that it will definitely rain.
Pay special attention to the scope of negation when 必ず appears. 毎日練習すれば必ず勝てるとは限らない。 means that daily practice does not guarantee victory. It does not also claim that practice is pointless.
Comparing the focus with わけではない
高いからといって、良い品とは限りません。 means that a high price does not justify a general conclusion that something is good quality. A noun or na-adjective can connect directly, as in 良い品とは限らない and 静かとは限らない.
この品が嫌いなわけではありません。 corrects a possible interpretation that the speaker dislikes this item. とは限らない focuses on whether a conclusion is guaranteed; わけではない focuses on rejecting an interpretation, although their uses can overlap in context.
Quick check: Does 安い店がおいしくないとは限らない。 let you conclude that all inexpensive restaurants serve delicious food?
Show the answer and why
No. It says that inexpensive restaurants do not necessarily serve bad food, leaving room for good ones. It neither praises nor condemns the food at every inexpensive restaurant.