There are also other Third party firmware available like DDWRT or Fresh Tomato so lots of choice 
I enjoyed reading your elaborate explanation.
I might have some clues regarding your final remark, ”… so the real question would be, why on earth does anyone buy bcm47xx hardware (NOS or not) in 2023…?!”
1- A new, non-dev user could easily fall in for a bcm47xx hardware which he has discovered in OpenWRT listings and learn about the shortcomings the hard way.
2- In my opinion as a home user, the “first best thing” about Linux was to resurrect an old PC struggling to run, e.g., Windows Vista or an old home-network router with nothing interesting out of the stock firmware.
3- While in many places on the earth, a decent router could be purchased by sparing one dinner in a restaurant, the same decent device could cost a whole-month income of most people in some other places. Then they have no choice, but to look for old, second-hand stuff and hope for the OpenWRT miracle.
So, for the sake of the unlucky cases mentioned above and in line with the technical excellence known from OpenWRT, it would be nice if any drawbacks, not just about RAM, but regarding any hardware feature of all devices supported by OpenWRT could be included in the ‘toh_extended_all’ page.
On the device page, yes (and it's already very explicit there, "Unsupported Functions: openwrt only sees 128MB RAM, WiFi 2.4GHz partly, WiFi 5GHz"), toh_extended_all not really - it's just getting too specific here.
I know, choice is good.
Still, if you want range and performance, nothing comes close. Merlin/Johns fork will route at near gigabit speeds (wired).
It’ll do 5ghz @ 220mbit (at 2x2, might be slightly better at the full 3x3 link speed).
The (near) gigabit wire speed is only if you use CTF (the Broadcom hardware offloading) which is not compatible with e.g. SQM, it is also available in DDWRT not sure about Fresh Tomato.
I personally would like a router with a Kernel which is still supported (being SLTS) than an EOL kernel 2.6 but that is just me.
Like I said lots of choice
But still we are talking about old and under powered hardware.
I am very happy with my Linksys EA8500, Netgear R7800 and Dynalink DL-WRX36
That goes without saying, you’re stuck using it as a «normal» router (although you do have some extras included in asus-wrt/merlin) and given the circumstances I think that’s the best choice.
Tomato and dd-wrt also use ctf and the proprietary broadcom drivers, however they can’t hold a candle vs the Merlin firmware.
Which means that even if you use DD-wrt/Tomato with their bells and whistles, the end result is that that Merlin will smoke them any day.
I remember flashing my Netgear R7000 with xvortex which was simply a bootleg Merlin build for that particular asus router. It was outstanding and I set up a few of these at various friends, who still use this with 100% uptime.