Kdump's backup (in kdump terms, it is "Capture Kernel") kernel is with minimal set of features and modules (scsi drivers, network drivers etc.) to have small memory footprint and resources to just handle crash dump related things. So,it's not a full replacement of primary kernel. Also, the point is not to make boot when crash occured. The idea is to take control when a crash occured by backup kernel without any need to reboot.
How is the backup kernel minimal? It is usually the very same kernel as
the "primary" one. You can use the same initrd as well and do a full
multiuser boot.