Just as there is no valid proof that increasing the block necessarily leads to centralization. 

But we have excellent proff that the current block size leads to centralization for users.

Millions of new users can't use the blockchain because they simply don't fit in. And to use bitcoin, they must cooperate with custodial services. They have no other option. Coinbase, with 25 million users, demonstrates this perfectly. 25 million !!! This means that half, or even more than half of users, use bitcoin through a centralized service.

Bitcoin is turning into a network of "bitcoin banks", where users trust their funds to them, and the blockchain for transfers is mainly used by"banks".