A key thing is that centralization / distributed-ness is fundamentally relative, and is defined in terms of something. Are 100 nodes, scattered in every part of the world and on every continent, decentralized? They are with respect to geography. But what if they're all owned by the same dude? Then they're not with respect to ownership. What if they're all running Bitcoin Knots? Then they're not wrt software. What if they're owned by 100 random people, but they're all Americans, or they're all employed by Square, or ...

You get the point.

Even going back to the idea that the nodes are spread evenly across the globe, in a sense, they're still not decentralized wrt geography bc they're all on the same planet. This seems a stupid distinction, and at the moment it is stupid, because we're all living on this same planet and their location on this planet makes no functional difference to any interaction someone would have with them. But it's not hard to imagine a context in which having everything collected on the same planet would no longer be decentralized from a relevant functional perspective.

This is what's so important about your last point -- whether some entity or cohort can achieve some goal (blocking something for everyone, in your example.) Decentralization (across the relevant functionality) is a state of organization that makes attacks of that kind difficult. It also makes development, deployment, testing, etc. slow and difficult. But the network lives with the latter drawbacks to realize the former benefits.