The risks are the same whether you keep running with a failed disk vs. rebooting/booting with a failed disk. Also, given a system in which the OS is not running

The risks are the same whether you keep running with a failed disk vs. rebooting/booting with a failed disk. Also, given a system in which the OS is not running and requires booting, how exactly are you going to resync a replacement drive if you don't boot the system with a degraded raid volume?