I completely disagree.  Mining isn't free.  Therefore once the block reward disappears & assuming no block size limit, miners will rely on transaction fees to pay for equipment, electricity, etc.  There may be some miners that process free transactions, but they will most likely have small hashrates & find blocks rarely.  The miners with higher hashrates will require a fee to pay their expenses & increasing your chances of getting into a block sooner.  We may also get to a time where trying to include every pending transaction in a block increases your orphan risk because it doesn't propagate fast enough.  Your orphan risk is the only block size constraint necessary.  And this will fluctuate naturally with bandwidth improvements.