Re: "Alternatively they could revamp the malware to the point where it's undetectable."
I am sure they already have revamped it and are now many iterations ahead. From the look of things they were after timely intelligence at that given point in time and presumeably they got it. Any further info from those sources would be mere background noise now and they would move on for legal, financial, operational or other issues.
I am sure that more sophisticated versions are out there and by leaving obsolete kit around for people to find they are muddying the waters and obfuscating other nasties that they have unleashed,