The one big battery is easier and faster - in large part because you can't lift that yourself so you have machines to do the work for you. For AAA you could design the same machine, but odds are you didn't.
If I had to replace 10k batteries, I'd probably make (or buy) a machine for that too. However, even replacing 10k batteries by hand probably costs less in employee hours than a single one-off machine for either example.
(40 hours / 10,000 batteries = 14.4 seconds per battery; USA minimum wage = $7.25/h = $290 per 40h week, about the same as the cheapest hydraulic lift I saw on machinemart.co.uk at current exchange rates, but I'm not a mechanic so I don't know if that would be the right tool for the job anyway).
And of course, the same applies if we really had drone swarms — anything like this would be automated, up to and including full disassembly of damaged units so their parts could be reused for new units, e.g. https://youtu.be/pDZdnbI0MAc
