The problem a lot of people in low Elo fail to see is that DBlade does more than just provide sustain. It also provides additonal HP buffer. It doesn't sound like much, but sometimes having 760HP instead of 700HP can make all the difference between a mid laner killing you 100-0 with their combo and you living when he 100-10s you instead.
Of course, in addition to that it gives you sustain throught the game. That percentage lifesteal really pays off after you get your IE as an ADC.
LS + 3 pots is a nice start on melee champs who can't make use of the sustain passive (and melee champs often have higher base HP anyway, and want to build Hydra or BotRK) but on ranged champs the right choice is usually DBlade, even if LS + 3 pots might be slightly better before the first back-to-base.