So get there while you still can, GrapheneOS is awesome :)

Given that the goal of GrapheneOS is security OS, and not the ability to run it on anything and everything, I would expect them to phase out support for old devices very shortly after the official support stops — primarily due to the fact that security fixes for those devices won’t be coming.

Precisely. Same reason why Apple pushes zsh and ancient bash down everyone’s throats.

TL;DR: they want more of your data