We will support our devices as long as we can. Of course we rely on the community for that; but if it succeeds for example for the sony xz1 (issued in august 2017), why wouldn’t it work for other devices as well, especially FP4 which is made to be durable ?
And even if a device cannot be upgraded to the latest LineageOS version, as actually LineageOS supports its 4 latest versions, this device could get the security updates of its latest Lineage version for 4 years…