It's a liability issue - they have to fix what they broke.
(The system boot really doesn't matter, it has to go into download mode. If necessary they'll have to do a JTAG reflash of the entire OS. They'll have to - it's their liability unless you got a notice to make sure that you had at least 50% battery and were given the choice of not updating [which they don't do - after a fixed number of requests, they just force an update to the phone, whether it has enough battery or not, causing a lot of dead phones].)