Given how badly Yahoo seems to be doing financially, and considering other high profile "failures" like Parse being shut down by Facebook and Apple being challenged by the state: I wonder whether these big companies can continue to extract rents after open sourcing and retrofitting everything with decentralised protocols, encryption etc.

That seems like a preferable direction from a consumer point of view than forcing society to rebuild everything from scratch, despite the fact that things like GitLab, blockchain and p2p social networks are not inherently inferior. Adoption is hard. "Charity" is the same as PR, so there might be more side benefit than the original speculators hoped for with the failed centralised business models by opening up and decentralising.