I think "centralization" is pretty ambiguous. Is Bitcoin centralized? Full nodes can be run on home PCs, so verification is decentralized, but only ASIC mining is profitable, so mining has become highly centralized. The Lightning Network adds additional roles, like payment hubs and watchtowers, with varying degrees of centralization.

So we can't characterize Bitcoin as a whole as centralized or decentralized. We should talk about specific roles, and the various attacks that could be performed by coordinated members within each role. Like we could talk about the possibility of Bitmain coordinating a 51% double spend attack, or the possibility of a major LN hub bribing watchtowers to ignore foul play.