Methodologically speaking, you are wrong here. Not every argument deserves a dialogue. A dialogue can take place just when compatible discourses are involved.

In this context, saying that bitcoin is doomed to centralization because monopolies and oligarchies are inevitable is just unproductive and useless and we don't need to spend our valuable time to disprove it.

On the contrary, engaging in such a melancholic discussion gives ideas like this a credit that they don't deserve, even if you manage to disprove them.  

Cryptocurrency is a movement and has an agenda decentralization being in its core, it is ok to question the technology's approach to decentralization but being a nah sayer who has an ideological prejudgment regarding decentralization as an absolutely impossible idea (and worse, a useless one), well, it puts one out of the crypto sphere and renders any discussion with him a useless time wasting practice.