I think it will have the Gab effect. Gab advertises as a free-speech service. However, those that are already in the mainstream are generally on Twitter. Thus, the people that are on Gab are the people who got kicked off of Twitter, and and thus Gab is in general a cesspool.

The decentralized services won't be able to provide as smooth a service as the centralized ones, if for no other reason than network effects. Most people don't really care about end to end encryption. What percentage of WhatsApp users or Apple Messages users will quit the service if Facebook or Apple dropped end to end encryption. It will be pretty close to 0%. Thus what you will have is that the majority of people on the decentralized services are there because they are doing shady stuff. Thus the decentralized services will become hives of scams, dick pics, terrorists, child porn, alt right, etc. After a while, even being on one of those services will be seen as suspicious.