Some years ago I wondered how long will it take them to go they way sourceforge went. Once you grow too much without a proper leader, you will fall :(.
Comparing GitHub and SourceForge as if they were cut from the same cloth is laughable to me. SF has always been a wretched hive of ads and dark patterns.
Sourceforge died in a very different way though. Bundling spyware/crapware in install packages for open source software was a serious breach of trust, and was pretty much the direct reason for mass migration to Github. Github is failing on the technical side, but they at least mostly have their brand value intact. I think it will still take quite a lot for a mass migration of the same scale to happen.