We don't want decentralized systems, we want small ones

We don't want distributed systems, we want small systems. It doesn't matter if they are centralized inside: what matters is that they stay at a manageable size. That makes them cheap, easy to maintain and almost free of moderation. One example is tildes, shared Unix servers where every user has their own account. They run on a trusted admin and donations. When they grow too big, more tildes appear. Another example is Friend Camp, a community of friends. It costs about $30 a month, around $1 per person.

Despite being centralized, they feel like the opposite of big platforms. There are no commercial interests and people know each other. It's like a shoe shop in a small town. The shop can only serve a few customers because there is little space, you know the owner's name (maybe even his wife's) and the shelves don't offer much variety; and yet you know you will be well served and the owner wants you to leave with what is best for you. In a big department store, on the other hand, the staff don't care whether you come back or not. The owner might be a board of shareholders who only want to maximize profit. Space is unlimited, but there are queues and it's hard to get help. You don't even know if the product is what you are looking for, even if it is cheaper. Welcome to economies of scale. Now apply that to centralized systems.