Offer-based installers are one way of monetizing and have become exceedingly popular, especially with closed-source freeware. Unfortunately, most of these installers are designed to trick users into installing (or go so far as to install without use consent). To SourceForge's credit, they make a single offer which is clearly defined and even have a link to a page explaining why the offer is made. Compare this to your typical download from Download.com which includes multiple offers including several tricks to get you to install (pages that seem to be license agreements for the main software but install adware when you click agree, pages that list a standard and a custom install type and you'll get adware unless you pick custom and uncheck the offer, etc).
The other option is freemium pricing, free for smaller users but bigger users pay. To go this route, you basically need to go after the enterprise to make money. Github has taken this route from the beginning and achieved profitability in 2009 if I recall. I'm not sure if Github is still profitable, but they got $100m in funding last year, so they have money to burn. That could be why the added binary hosting back in. They had it originally but removed it due to costs years ago. Still, I'm unsure if they would put up with a project as large as the ones on SourceForge (PortableApps.com, FileZilla, GIMP for Windows, etc) as that is quite a bit of bandwidth to give away for free as a loss leader (since Github wouldn't even make money on ads for those downloads). Unfortunately, SourceForge has no enterprise offerings at present, so this isn't a viable revenue source today. They could explore this route in the future as a competitor to Github, though. It would be a bit of a pivot for them and not without its own risks. But, even if they go down this route, they still need to have the revenue today to keep going. So, it's a bit of a catch 22.