I sort of agree. In fact I mostly agree, you either score or concede, or you don't. That's all that ultimately matters.
But I do find it interesting that Cardiff were highlighted as overachieving on xG last season, did nothing, and ended up in a serious relegation battle.
Similarly, there was a minority of people at the time during Monk's 8th place season who were adamant that we had got lucky and he'd completely fucked us up. At the time they looked like moaners, but they turned out to be right. The drop off the following year was dramatic. That's because he didn't change and the luck ran out.
There's definitely something in it as a metric. I don't think it should be the be all and end all though.