The decentralized web is already a "successful" idea. The correct implementation for its wide use is not there yet. But it will be there.
It is just a matter of time before we have a bigger "dark web", a decentralized web, decentralized payment networks, and still have Google, Facebook, and the likes.
As the internet population grows, and as people move to more digital lifestyles; the people won't be limited (or gravitate) to a single portal. Instead, they'll spread over different networks/infrastructures for their different needs. Facebook can still be successful and grow while the decentralized internet happen.
The Internet is growing both in number (population) and in use. People today use the Internet to surf, chat, read the news, buy stuff online, book flights and hotels, pay taxes, work, study, find partners, buy drugs, etc...