>I've had people tell me that nobody should use anything but GrapheneOS and stop supporting alternatives to throw all support into that because the others are "less secure"

Without having an kind of authoritative knowledge or experience on the topic, those people are wrong and please ignore them. The argument has generally been that if you are specifically after privacy and security in your personal device then GrapheneOS or post-MIE iOS will be your most sensible choices. You CAN choose devices for other reasons, as has always been your prerogative.

The question of whether to support 'alternatives' is fraught. It used to be that there were two other OS projects that happened to be collaborating and adopting features from GrapheneOS and that would have been reasonable. The main argument (from GrapheneOS) in that case has been for people to please invest in alternatives with approaches to privacy and security that stand up to threat-model driven design and real world attacker/defender experience.

GrapheneOS was never meant to be alone in pushing for things like hardened secure element-based protection of secrets and side-channel resistant rate-limiting of unlock attempts, memory tagging/hardened memory allocators/secure application spawning/dynamic code loading control, anti-persistence hardening, prompt security patching, network/sensor permissions, contact/storage scopes, PIN scrambling, auto reboot etc. Unfortunately very few other projects that I am aware of are looking into doing things like this to give the device owner control and mastery over their data.

>and now that GrapheneOS isn't for everyone and anyone -- the majority of people -- without a specific narrow selection of hardware should get lost.

GrapheneOS tries to make most of their hardening transparent and non-intrusive by default. They also spend a lot of time and resources working on usability (sandboxed-Google-play and the web installer) and now accessibility (upcoming text-to-speech implementation?). The idea is that if you have a Pixel and choose to use GrapheneOS then it should be as easy to use as they can manage without compromising their efforts improving privacy/security. In that sense, GrapheneOS is for anyone and not just security nerds or tinfoil hats.

The exclusivity to Pixels is an unfortunate consequence of being the only platform equipped to provide what they need to achieve their goals. If multiple devices supported what they needed from the beginning, they would have probably supported three or four models from different brands as targets (for example you could imagine a couple Pixel lines + one Samsung line (Europe/North America/Oceania), one Xiaomi line (East Asia/South East Asia/South Asia/South America), one Tecno line (Africa). This is speculation on my part, but the main point is that the Android OEMs have been seriously slacking on basic privacy/security leading to this kind of situation.

>We need the people who buy $100 phones to have the ability to put a better OS on them than the burning mudslide that comes with them, is all I'm saying.

No disagreement here. This relies on AOSP adopting improvements and also on Google tightening their certification (for Play Store) requirements to include stronger privacy and security guarantees.