To make things worse, sensors often only transmit a new value if it changed w.r.t. the previous one. If an isochronal sensor skips a sensor value at the gateway, we'll never know if it just didn't send the value because it was identical to the previous one, or if a new value wasn't received.
That's where there's value in numbering your packets and having the gateway keep track of how many aren't received. Your statement is still true, but at least over time you develop statistics about how (un)reliable the connection is, so you can be as pro-active as you want to be in fixing it.