The vast majority of users do not use a strong passphrase. The recommended high security setup is a strong passphrase and 2-factor fingerprint+PIN secondary unlock for convenience. Using a weaker PIN for secondary users for convenience is not our recommended approach.
MTE is required for the majority of the additional protection provided by GrapheneOS against memory corruption exploits. Nearly all remote exploits and most local exploits involve memory corruption. MTE is only going to become more important as we implement deeper integration for it.
As in: people who do care enough to understand the technical arguments tend to go for GrapheneOS when they can. But if you build a "degraded GrapheneOS", you are not targetting those. For someone who doesn't care about what GrapheneOS brings, why would they use your system versus /e/OS?
DivestOS was a thing at some point, which was technically very interesting. But there wasn't much of a differentiator since the people who already cared about what DivestOS was doing were probably already looking at getting GrapheneOS.