Why? The minimum fee to relay a transaction is far less than the minimum fee to confirm a transaction in a reasonable period of time, and Bitcoin Core already calculates the latter automatically, which is all real users care about. Anyone who overrides this recommendation to pay the absolute minimum should "be aware that this can end up in a never confirming transaction" to quote the warning given when you attempt to do this. Bitcoin is not, never has been, and never will be suitable for microtransactions, nor was it designed or intended to be.