Extended support actually matters no matter what OS you use. Firmware update can only be pushed by the manufacture, not even fairphone themselves can update the chipset firmware without Qualcom, let along lineage or calyx.

This is also why security-focused OS, like graphene and calyx, put phone in extended support after the OEM drops support for the phone. Although they can keep android up-to-date on the surface, the underlying firmware will no longer be supported with security updates.

This is why fairphone 5 picks a industrial chipset with very long support terms. So that your phone can stay secure for longer period of time.

The SOC used in the Fairphone 5 will receive 5 years of support from Qualcomm this will also benefit custom OS'. And the FP5 isn't just about longer support it's also about repairability, sustainability and good working conditions. Yes it's expensive, but so are other high-end smartphones that are even more expensive and only last 2 to 3 years.