opkg's memory was (still is) one of the reasons for this strong recommendation, but by far not the only one. Even if you don't use opkg at all (up to the point of not having it installed in the first place/ custom build), 32 MB RAM is somewhere between marginal and beyond the edge. Sure, you can find use cases where this might not be an issue yet, but most users will feel memory pressure and "unexplainable" crashes sooner than later. Keep in mind that especially sysupgrade also needs quite a bit of free RAM (flash size plus some safety margin, with devices sporting more than 4 MB flash this is already a serious problem) to operate, with quite fatal consequences if RAM runs out while flashing. PPP, dnsmasq, wlan drivers (hostapd/ wpad) and the webinterface (uhttpd/ lua/ luci) plus eventual tls providers (mbedtls, cyassl, openssl) also need their share of free memory, combined this is hard at (or slightly beyond) the limits 32 MB RAM impose in many of the most common use cases.
Contrary to your very hand waving suggestion, PPPoE is a hard requirement for pretty much all ADSL or VDSL users, many fibre setups also depend on it (the same for all kinds of mobile-/ cell data in the form of pure ppp) - the only users which basically don't require any kind of ppp are cable based or use the LEDE device behind another (ISP-) router. The situation is pretty similar for IPv6 support, with many ISPs switching to dual-stack and particularly various DS-lite variants,because they've run out of IPv4 addresses (this is common for many cable and fibre users in europe already).