Well, you're attacking a straw man a bit here...the argument generally is that using the phone as a mounted bike computer drains battery excessively fast, not that using your phone to track your ride drains battery.

Also, I'm not sure why there is a need to "debunk" the "myths" about the benefits of using a standalone bike computer...I don't think anyone is saying they're necessary, but simply that they have certain advantages (and yes, battery is one, since my Garmin with display on would have about 50% of its battery left after the ride you mention, and my phone would be almost 100%, without needing to carry around a separate battery and cable) over phones, and some disadvantages as well. But hey, if you don't want/need one, then don't buy one, and feel free to tell others why you made that choice. We're not talking about vaccines here....