While you are correct a global power outage may render BTC temporarily unusable, as soon as normal power is restored the system would quickly resolve itself to a new chain and continue on.

If transactions did happen on a fragmented chain during the crisis, they may be reversed once everything returns to normal. But anyone who understands blockchain would either require an extraordinarily large number of confirmations during the period to try and mitigate this, or simply stop accepting it altogether.

In the case where islands of fragmented chains were occurring, it's likely people would be engaging in out of band communications to determine which chain represented the largest difficulty, so after a short period mining and transactions would die out in all but that one region, and on the valid chain people would be extraordinarily careful to calculate when the transaction reached sufficient difficulty. There are enough people who understand the math involved that some commerce could still take place.

All that said, given StarLink and the number of independent miners with access to offgrid solar and wind power, it is exceedingly unlikely that a worst case scenario of fragmented chains could even occur in practice.

The point is, unless you think we are permanently going back to the stone age, if it occurred at all, it would be a very fleeting and temporary situation at best.