>Decentralisation - at it's core - is about ownership. If you put content and data on a host, who owns that? You or the owner of the host? This is where the entire discussion becomes a legal debate.

Your analysis that "ownership of data" being the core tenet of decentralization is incorrect. Consider the 3 major examples of centralization complaints:

- Youtube is too centralized. A decentralized alternative is PeerTube. Is this about ownership? No because Youtube's TOS specifically says the uploader to Youtube retains ownership of the content.[1]

- Twitter is too centralized. A decentralized alternative is Mastodon. But it's not about ownership of data. The Twitter TOS says the user retains copyright ownership of the tweets.[2]

- Facebook is too centralized. But again, the various decentralized alternatives do not change the terms of content ownership. Facebook TOS says the user keeps ownership.[3]

What the 3 examples show is that data ownership isn't the issue. The real issues of centralized services are power and audience reach.

If a musician writes an original song and films an music video, and then uploads it to Youtube, the musician retains the data ownership and the intellectual copyrights. Because intellectual property ownership wasn't transferred, Youtube can be seen as a glorified content-delivery-network cache. (And there's the rub... that so-called "cdn" of Youtube has attracted a billion eyeballs.) That means there are different issues that motivates decentralisation; e.g. the power to filter/rank/recommend/censor/demonetize videos. The concentration of massive audience reach in Youtube's centralized service that makes the musician's decision to host his music video on his own home server irrelevant.

I think those 3 examples should trigger a rethink of your mental framework that led you to conclude data ownership is the main issue.

[1] excerpt from https://www.youtube.com/t/terms : 6.C - For clarity, you retain all of your ownership rights in your Content. However, by submitting Content to YouTube, you hereby grant YouTube a worldwide, non-exclusive, royalty-free, sublicenseable and transferable license to use, [...]

[2] excerpt from https://twitter.com/en/tos : You retain your rights to any Content you submit, post or display on or through the Services. What’s yours is yours — you own your Content (and your incorporated audio, photos and videos are considered part of the Content).
By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, [...]

[3] excerpt from https://www.facebook.com/legal/terms : "You own the intellectual property rights (things like copyright or trademarks) in any such content that you create and share on Facebook and the other Facebook Company Products you use. Nothing in these Terms takes away the rights you have to your own content. You are free to share your content with anyone else, wherever you want.
However, to provide our services we need you to give us some legal permissions (known as a ‘license’) to use this content. This is solely for the purposes of providing and improving our Products and services as described in Section 1 above.
Specifically, when you share, post, or upload content that is covered by intellectual property rights on or in connection with our Products, you grant us a non-exclusive, transferable, sub-licensable, royalty-free, and worldwide license to host, use, [...]