What if the bottleneck of restaffing isn't training time but available labor pool? If Middle America Town #4 already has a majority of the young, able bodied labor force employed by Amazon then Amazon's leverage is heavily reduced; there may not be enough people to restaff.
But the fulfillment centers aren't all that remote.
Assuming the list posted here [1] is accurate, they tend to be located on the outskirts of cities. Looking at Virginia (where I live), the only one that I'd really consider rural is Clear Brook, but these days, that's effectively a DC suburb, with plenty of potential workers in nearby Winchester VA, Hagerstown MD, and the I-81 corridor.
Are they though? If Amazon wanted to stick it to the Unions, they could offer similar terms to the new scab workers and not to the striking union members.
Amazon can train these workers very quickly (i.e. 1 day) so it should not be very hard to restaff a whole warehouse.