I think ultimately they are on-topic—I certainly don’t think they cause any kind of problem that should lead to our deciding otherwise—but I would comment that Stack Exchange tends to not be a great format for performing optimization. The format works far better when an answer can point to and summarize the optimization work already done elsewhere. This is because an expert in the subject may easily be familiar with optimization work already done, and is in a good position to find and summarize it, but even a subject-matter expert here has no way of ensuring that a given approach is strictly optimal—as you say, there can always turn out to be another option to bump things up, or a better approach that hadn’t been considered. Optimization is often a collaborative, iterative process, and Stack Exchange doesn’t really do that very well.
Stack Exchange is, in this regard, far better at finding a good answer than it is at finding the best answer. That said, frequently a good answer is more useful—because finding the absolute best is hard. To block these questions on that basis would effectively be a matter of siding with the perfect against the good.