1. To start, if decentralization does not "solve the problem" then what do all AP platforms bring to the table? There was already a decentralized facebook before mastodon - it used to be called a "web site". You could get a feed with just what you wanted by subscribing to rss feeds, which were also decentralized. You were not tied to 140 characters per post either, and nothing stopped someone from making an rss client that clipped posts at 140 characters in the the feed or whatever. What problem does AP solve for me? Indexing all that content? The hosting? If the whole thing is about getting banned from the domain twitter.com, open your own domain and publish your content - problem solved.
2. If we're solving a solved problem, why not bring something new to the table? eg anonymity of nosr.
3. If "decentralization is not the solution", how come other protocols - eg mentioned above - claim (convincingly imho) to offer an actual improvement if not solution?
4. You claim "decentralization is not the solution" then define some ad hoc line in the sand regarding what is "decentralized" and then make some bold claims based on that. Decentralized could mean many things: could be like ssh keys, could be like gpg keys with directories and web of trust, could be like dns or certificate authorities. Could be like email. Like torrents or like i2p, or kad. All of these are slightly different. Heck, could even mean like fb vs twitter - you are free to move to a different platform. Takeout your data, import contacts - done.
Where you claim that decentralization is not the answer i will say maybe insufficient decentralization is not the answer - with email, AP, and many other protocols, the platform ("domain") is the first class citizen so you get strong platforms and weak users. Regulation will help here just like it helped with banking - ie wont. What makes banks cooperate is mutual desire for profit.
Banks are truly fungible for the most part - make users first class and you will get the same for platforms