This technology already exists (NOTE: My understanding of it might be incorrect or limited as I'm not a developer myself). Essentially, when using WebRTC, the transferred data does not need to be saved on a server, but there has to be a relay server to connect the two computers. That means the users themselves carry the cost of storage and data transfer. Copyright infringement or other illegal material would not be an issue either as the service provider is comparable to a phoneline operator.

Commercial comparables include ToffeeShare (EU) & partially Wormhole (United States). Wormhole saves files smaller than 5 GB for 24 hours in order to enable the ability to pause and resume. ToffeeShare does not save any data, which means the transfer must always be fully completed or it must restart from zero. This can be an issue for large files and unstable connections. Both are closed source, so one has to take their word for it.

Many FOSS comparables exist, such as Magic Wormhole & croc. These are perfectly functional but don't satisfy an average person's UX expectations.

Such a service would likely not be a revenue driver, but would make a useful addition to the Proton toolkit.