Hang on a second. Am I missing something? I don't think miners need a hard block size limit to have incentive to stop accepting transactions. They will do so because there is always a time limit.
The difficulty target is adjusted to regulate time between blocks, and results in a target with a probability a correct hash will be found within a certain time (regardless the total hashing power of the network). Every second a miner waits to include more transactions in their block the probability is increased a competing miner will find a correct hash for their own block.
Yeah you're missing something.
Miners are always constantly hashing to try to find a new block that would include the most profitable set of transactions that they could include in that block. If they find a hash that meets the target they should immediately send the block they found to the network and start trying to find a hash that would build on the block they found.
Under no circumstance does it ever make sense to withhold a solution. If they find two blocks in a row, splitting the transactions they could have included in the blocks between the two blocks, that's actually better for the miner because it makes it harder for other miners to orphan those blocks, and thus collect the fees themselves.
You have to remember that mining is a random process. It's not like you work towards solving a block, it's more like you have this machine that spits out lottery tickets, and you are scratching them off as fast as possible hoping for a winner. You might get lucky and have two winners in a row, or unlucky and go for days before finding another one, but either way ever winner you do find you should cash in immediately however much it's worth.