I think that is ultimately the answer, which is: Yes bitcoin will eventually (unfortunately) undergo massive centralization. As the masses join the system, control will cede into larger and larger silos of centralization.

However, much like the modern internet, it will still be possible to use bitcoin in a pure P2P fashion. At the protocol level, bitcoin will probably remain 'decentralized', its just that for convenience purposes, >95% will choose to use it in a completely custodial fashion.