Is centralisation necessarily a problem?
For now it's more of a concern. It's all about potential. Centralised points of failure aren't generally perceived as a problem
until they fail. So it's better if something could be done to encourage a wider distribution of the load. But what that "something" might be, I sadly have no idea. Hopefully it's something that happens organically through market forces and competition, but there's no way to tell right now.
Something things do have to go wrong before people adjust their behaviour. When mistakes happen, some people learn right away and change their habits, while others don't. But gradually, over time, there are fewer central points of failure and the whole thing becomes more resilient. A similar situation could arise here. If any of these major nodes have failures, people may be more likely to use smaller nodes in future.
All I can suggest is keep a close eye on it, see what develops, then make contingency plans if it looks like it's getting worse.