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.

When the network receives more than one valid block version within close time proximity it holds both and waits to see which is extended longest breaking the tie. Again, every second a miner waits to announce a block it increases the chance their found block won't be permanently regarded as valid. Physical block size is not a factor in that, and miners will naturally stuff their block with most profitable transactions first.