@bob I think it's more appropriate to define decentralization in terms of power structures.
Look at Bitcoin: it was supposed to be completely distributed, but then, 10 years later, a single table of 5 people represented 90+% of the whole mining power.
If you look at the Fediverse, and especially at where do the servers run, you might end up with a majority of Amazon servers... That's probably not what we should be calling decentralization.