>however using an old PC as a router almost certainly wastes an enormous among of power

I don't think you're quite right on this, or at least you're imagining using something inappropriate when the comparison here involves buying something new right? So it's not "OpenWrt One" vs "whatever you happen to have in your closet" but "OpenWrt One (~$110-130)" vs "whatever can be bought used for $110-130, if you have nothing appropriate". And while they won't go to near-zero like some ARM stuff might, idle power for PCs improved a ton after around the 2013 era. There are lots and lots of small systems available for equivalent prices on Ebay or the like made since then (like Intel NUCs or various other mini PCs) that will idle around 4-10W. Like to take something in the same price range as this OpenWrt One, I regularly see 7th gen era NUCs going for <$140. An i5-7260U will have single threaded performance about the same as the MediaTek in this unit and multi-thread close, but will also generally have 8-16 GB of RAM and often a 250-500GB NVMe drive as well. It'll probably have only one native ethernet, but USB or TB adapters work fine with Linux & FreeBSD at this point.

There's definitely a question of values and exactly what you're trying to focus on, but there are a lot of niceties in having lots of RAM on tap and extremely standard fallbacks to interface with a system, back it up, etc.

>or an OpenWrt One which will possibly also have newer, faster WiFi standards (WiFi 6)

If you want an AIO style device that's definitely a consideration, though again USB WiFi dongles are a thing too. But regardless of router choice, for someone considering going beyond what their ISP offers at all I think it's usually well worth spending the $50-80 to get a dedicated wireless access point. It'll make a major difference in real world performance in most spaces I've seen to just physically have a unit in an ideal spot (on a ceiling or high up on a wall, away from metal). Aesthetically the clean disks or rectangles those tend to have also blend well and mean that various boxes can be tucked away. And of course you get to upgrade networking bits separately from your router.

Anyway, definitely good there are multiple approaches, this is an area of life where people can have very different needs driven by very different physical environments and "stakeholders" (like significant others). But I think OPNsense (or other bog-standard-PC FOSS alternatives like VyOS) can be competitive even in TCO, depending on how much value you place on pushing your networking stack and what else you have going on (like solar power).