Decentralization doesn't imply you grow all your own food. You could still purchase your food via multi-trillion dollar markets, but the suppliers to the market could all be independent farmers, for example.
Trade and specialization are completely orthogonal to the centralization <-> decentralization spectrum.
Hmm, i don't know. I think the increase in specialization means the ability for "the one single best" actor. Like how NYT is one of the only thriving newspapers as many collapse, or how fewer companies make a bigger % of the cars in the market
However, trade also increases output, and with it, the diversity of goods/services.
60 years ago, television media was dominated by three networks. Today, there are dozens of cable networks, and millions of creators who broadcast through video sharing sites like Youtube. So there are many more sources of video-based media, and that is owing to greater economic sophistication brought about in large part by efficiency gains through greater commercial-exchange/specialization.
In this specific case, I'm making the claim that independent farmers have been and will continue to be on the decline. And in the general case, in the case of trade+specialization, it seems that over the long run the "best" at the trade will eventually own an outsize amount of the market. Technology enables them to win.
This seems by observation to be true in distribution, in farming, in content, in finance, etc.