stated
>There will be a penalty for not including a donation, but it cannot be the orphaning of the block.
I wonder if 'soft orphaning' might be an option.
Where a non-cooperating
block is created, cooperating miners would choose not to mine on top of it, but start construction of a parallel chain. Where a second consecutive non-cooperating block was created,
risk of permanent chain split * miners free to create block types they prefer with chance they will be included * persisent cost to those enforcing coop blocks if support is low (risk
tl;dr If even a slight risk to miner a block could be dropped then a non-zero contribution is rational. Feather forking like this also fixes difficulty gaming.