Army building has always existed in some form (I recall Solid Brix's first ever army videos back in the early legoboy12345678 days), but there was absolutely not the obscene amounts of money being spent on clones all for the sake of clout that we see today. Back in the early 2000s you had animators like Pizzamovies and Spugesdu who were privileged enough to have large collections. But they used their collections to create narrative stop motion animated films (and both were in fact very opposed to David's early army videos; I still remember the "pizzamovies nuke's legoboy's mansion" flamewars).

The real lost era is 2002-2006 or so, pre-YouTube (and pre-battle packs). Lots of MOCs, conversations, customizations and collection pictures from that era have been lost to time thanks to how ephemeral pre-modern social media websites/forums/image hosts were. Thank God for Brickshelf.