You are forgetting Merc treads were core on marksmen until season 3, and 'zerkers were seen as trolling on them for years.
Additionally, in seasons 1-2 they needed at least a BV or GA, or both, and situationally a QSS against Malz, WW, Sion, Taric, Fid, Morg, etc, as they all had 2 second or longer point and click hard CC or were Morg (Morgs ult was a 2 second stun on top of her 4 second root, she could chain CC one target for 6 seconds by Q -> r1 -> r2 triggers while rooted)
So, defensive boots + 1 guaranteed defensive purchase (2 defensive items)
Or defensive boots + 1 guaranteed defensive purchase + situational QSS (3 defensive items)
Or defensive boots + 1 guaranteed defensive purchase + 1 extra defensive items (GA + BV + mercs is usually how this played out against heavy dive comps, fed midlane APC/AP assassin, or Kassadin was on the enemy team before season 4. 3 defensive items)
Or defensive boots + 1 guaranteed defensive purchase + 1 extra defensive items + situational QSS (4 defensive items, this would only happen if you were public enemy number 1 on some hyper carry with passive damage, we saw builds like this only really on Vayne and Kog, but with a protect the carry comp, this wasn't uncommon in higher level. Also, its worth noting that these ADCs would not be purchasing much crit, but would generally rush BT (it needed to be rushed as it had stacking AD and lifesteal mechanics based off kills/CS) and then an attack speed item, and would get buffed up AD/AS from their support and team. Additionally, when GA's passive is down, it would typically be sold for a damage item, then repurchased if needed, when the cooldown expired.
Additional points of GA being an MR/armor item, BV being a mana/hp/mr item, and no build path for QSS, so I know this sounds utterly ludicrous, but ADCs used to actually have to give SOMETHING up, and actually try to make MEANINGFUL decisions
They didn't have cheaper items than literally everyone else, but actually the most expensive items. If they were behind, or getting dove, they needed to invest in defense because it was actually worth more to then to survive for a bit with lower damage, to at least be able to get that damage of consistently. Y'know, like an actual squishy dps class. Since the defensive items used to be 2600g they would be worth purchasing to make sure the ADC could survive long enough to start scaling up, or NEEDED THEM TO BE ABLE TO CONTRIBUTE IN TEAMFIGHTS AS A SQUISHY CLASS OF CLASS CANNONS MIGHT ACTUALLY NEED TO INVEST IN SOME MEASURE OF DEFENSIVE IF THEY DON'T WANT EVERY OTHER CLASS TO EASILY BE ABLE TO SLAUGHTER THEM
Huh, interesting, that's the price point for all their AS/Crit, and armor pen items now. . . funny that, and the tank items have increased in cost to now be where the only Zeal items were, and the melee fighter/juggernaut items are at the higher price points that used to be for ADC items. . . and we no longer see ADCs needing defensive items period, as they can instead just get the way more gold efficient, cheap, and much more impactful and "fun" damage items that let them melt literally anyone in seconds. Not to mention, they now get MASSIVE amounts of damage on any of their "defenisve" (fucking LOL) items for free, and extra tacked-on benefits, like massive % damage reduction on fucking AS/Crit/MS item that give phasing for 2,600g, or up to 2,000 free magic damage to champions 2-3 times a teamfight, or nigh instant wave clear to shore up the supposed wave clear advantage APCs had over ADCs. . . on another 2,600g AS/crit/MS item. . .