Yeah, except they don't actually lose any damage and Lifeline of MoM even gives you lifesteal AND spellvamp so shove your "oh wait" up to your ass because you are talking bullshit.
ADCs have to build IE, a zeal, a zeal or an essence reaver, a lifesteal item, and a last whisper item.
MercScem already significantly reduces the damage and lifesteal you have compared to other options, as shown by its purchase rate. Considering this staple build for an ADC, where is the room for a Maw? Do note Maw is garbage late game.
Now, considering these facts, how is an ADC not losing significant damage?
To your previous comment: Just played against Twitch as Lux. He bursted me faster than I would burst him, I had armor item, he had no MR, but instead he had GA. What could I do?
You gave too little information. What level were you? What level was he? How fed was he? How fed were you? What items did each of you have? What runes did you have? Was it early game? Mid game? Late game?
Also do note Lux is just a bad champion and you shouldn't expect much from her, similar to me with Karma supports.