When MS also announced they were closing that and offering a tool to migrate to GitHub, was when GitHub (and Git) truly became the biggest remaining option.
The other aspect of SourceForge's decline was that they doubled down on the sketchy site feeling right as acceptance of such sites was on the decline, the likes of mediafire, zippyshare etc were being replaced with cloud storage providers and download aggregators were losing popularity (in part due to also becoming very sketchy and prone to pushing malware). They might've been able to get away with it a few years earlier. I remember that back then it wasn't a huge deal to follow a mediafire download link from somewhere that seemed reliable enough, whereas nowadays it'd be an immediate red flag due to the abundance of more legitimate seeming file sharing options.