It seems that there are a number of blockers to widespread adoption. Some of those are:
1. Inertia. Most folks are already on centralized SN platforms, as are their contacts, friends and family;
2. Complex installations[0] keep many folks from even attempting to install such platforms;
3. Lack of symmetric, high-speed broadband access limits the ability to self-host such platforms;
4. Limited/expensive access to domain names, DNS hosting[1] and static addressing (in the absence of widespread IPv6 adoption by consumer ISPs).
Given that most consumer-grade computing systems are mostly idle and storage costs continue to plummet, the argument for decentralized platforms just becomes better and better.
As such, why aren't more folks promoting them and/or developing mechanisms for broader access/acceptance?
[0] Yes, there are Docker images for many of these platforms, but implementation/adoption of Docker is limited and most of these platforms require significant IT and development knowledge/skills to implement.
[1] This could also be addressed via containerization, but there seems to be limited interest in promoting such self-hosted platforms.
More importantly, you seem to place a higher value on decentralization than centralization. Why is that? What if centralization allows more people express themselves more freely? Would you place a higher value on centralization?
Decentralization is a design choice, that's it. Unfortunately, we have to come to terms with the fact that outside the sphere of hackerdom and a select few privacy-conscious individuals, decentralization is completely and utterly irrelevant.
You might, maybe, be able to get a federated system, whereby people could sign up for major providers that all talk to each other. And that could include being your own provider. For most people, that would be no harder than signing up for Facebook.
Now you just need to give them a reason, and no, nebulous "privacy" isn't going to cut it. Nor is "portability". If anything, the easier it is to move your data around, the more it's going to seem like picking one provider over another is a waste of their time.
The whole point of social networking is networking. People want to connect. "Private social networking" is a contradiction in terms. There's little motivation for people to take control of their social networking, and they're happy to put up with the mediocre privacy controls because the vast majority of the time they want to be more public, not less.
People could still "like" your photos and comment on them. Make URLs shareable and then they can email or what ever to their other friends to see if they feel like it.
I know, too much friction. But if they really cared about the content I would guess your friends would hop into the opportunity.
The concept of the contact list is primitive though. The open source community has neglected to develop tech for easily reusing it, and neither apple nor google have any interest in making it interoperable with other services.