As a reviewer, my goal is to find critical flaws, not to "improve" the paper. Every paper can be improved in 100 different ways, many of them subjective. The authors have already decided they're happy with the paper as is. A reviewer is not a co-author or a collaborator.

To clarify: the goal is not to "shoot the paper down" - it's to accept the paper as is, unless there is a critical problem.

The authors have been working on this paper for years; they've probably already thought about any suggestion to "improve" the paper, considered it, and decided against it. It should be their call.