it's a valid question, and I think people might be able to present valid arguments either way.
Argument for allowing treasurers on the board is that they are highly aligned to the foundation's purpose. However, like you say, there is risk of abuse of power and a potential to lack a diversity of opinion.
On the matter of centralization of power:
there remains to be seen how much power either role will have to act. Treasurers are stated to only act in accordance with whitepaper or DAO-authorized transactions.
If the board members are known and KYC'd, there might be some future scenario where a board member is pressured (either legally, or not) to pursue some action without authorization from the DAO. If none of the board members have wallet keys, it seems that might reduce that particular sort of risk.
Invalid concerns.
to you, sure, but clearly not to others, so more clarity is clearly needed 😀
all valid concerns