Even 128 MB may not be enough for a device with two ath10k radios. See, as one example, ASUS RT-AC58U crashes under heavy load/ after a while
64 MB devices are almost certain to have problems over the next three years, following quickly on the heels of the now-obsolete 4/32 devices. This is nothing new, and the RAM issues are already mentioned at https://openwrt.org/supported_devices/432_warning
I don't think that LuCI is a crutch for users who don't have the skills to select a device based on their knowledge of their needs and the resources required. By its very nature, a filtered list is targeted at relatively new users, for whom LuCI is a strong, positive feature of OpenWrt.
Ruling out MTK is a little heavy handed, I believe. The business model you describe isn't very much different than any of the vendors, chip sets or devices. FOSS has become a "religion" and, in my opinion, one should be as careful about pushing it on others as any formal religion. There isn't anything that is truly "open source" when talking about any IC with any complexity to it; SoCs, switch chips, and wireless NICs definitely fall into that category, even the much-touted ath9k devices. They all have closed-source firmware, be it embedded in the device, or downloadable. I haven't found an IPQ4019 data sheet (not sell sheet) available without NDA, as a specific example.
Several strong, candidate devices are mentioned at Top ten routers currently in use? and in posts following (as a redirected response to the OP's initial query). The omission of the MTK-based devices probably brings many in. Others, especially in the "cheaper options available" may not be captured by your list (the BT Home Hub 5 Type A, as a notable unit that is widely available in Europe at an excellent price, from what I understand). Some of the other "omissions" were apparently due to being single-band devices, which was not noted when I looked at the page.