Once Google stops tracking and fixing bugs in a device's firmware, the device becomes increasingly insecure over time, and also over time becomes increasingly difficult for the GrapheneOS team to maintain.
More work for less security is not a great mix, and at some point there is a risk that vulnerable people will think that the existence of an extended-support release means the device is still "OK to use".
Perhaps Google may decide to support the firmware for a little longer. Perhaps the GrapheneOS project may decide to issue an extended-support release for a few months. But I wouldn't plan on either one of those happening, and I would suggest having a backup plan ready. If it will take several months to import a newer Pixel device, then it is probably best to get started on that before the EOL date, or at least on the EOL date if it isn't extended.
Please note that I don't speak for the GrapheneOS project.