An upcoming report need not be final or signify the end of Mueller's investigation even IF final.
Nothing indicates, nor is there a requirement, that an upcoming report will be final or that it will end the Mueller investigation even if it is final......NOTHING! I read articles over and over, and the idea that a soon to be released report will be the final one or that issuance of a final report will end the investigation by Mueller's group is simply assumed by the writers.
There is nothing in the law that says there cannot be interim reports and nothing in the law says that the office must close down even after a final report. Most basic is the reality that people charged must be brought to trial, and while Mueller may hand off prosecutions, he does not have to do so. He may hand off some, while retaining purview over others, even after a final report, and that would be especially so after an interim report.
Maybe he thinks that Trump and crew pose such a danger that he must tell what he knows at this point to hasten the corrective measures necessary to protect the country, even if his group still has work to do and criminals to prosecute.
Plus, what about all of the data seized from Stone? That has to be thoroughly examined, and new prosecutions could be brought as a result. What if Stone or others flip? Much new information would come as a result of that, which would very plausibly lead to new prosecutions or the strengthening of existing ones.
Reporters are assuming a lot. Let's not let assumptions drive the narrative or our fears.