We would support proposals that change consensus rules if there’s overwhelming consensus to do so, but specifically in the case of opt-in we were imagining some flag for users (to enable in their transactions) and/or miners (in their coinbase transactions) that would determine whether funds are awarded to a potential future dev fund (and at what proportion to potential recipients). A scheme along these lines would definitely require a consensus change.
I understand the concern about complexity, but in our view this would preserve user choice and provide important signaling for the ECC, Zcash Foundation, and any other recipient of a future fund. Note that others associated with the Foundation may have more detailed or nuanced personal views (see @amiller’s comment above), but this was the consensus reached by the Zcash Foundation Board of Directors on how we’d approach the possibility of a new dev fund.