Why is there a need for "fee pressure" if miners can choose their transactions?

I don't get this "fee pressure" argument in nearly every blocksize discussion.I understand we want to secure the blockchain, especially once the block rewards get lower.But if miners can choose themselves which transactions to include in their blocks, why is there a need to set a blocksize limit to force "fee pressure"?Wouldn't miners decide for themselves which transactions to mine? Wouldn't they simply calculate which fees are worth mining, and simply reject anything lower?Wouldn't competition drive down fees to the equilibrium between what people are willing to pay, and what miners are willing to include?Please explain to me why I'm wrong. via /r/Bitcoin http://bit.ly/1MxaD6q