adcs are always going to be crowded out by whoever is the strongest. thats it.
its improved a lot, but still has a ways to go.
you see Ziggs in bot lane now, mordekaiser is still viable, and wildapsona says he can play ap sona in bot lane even in challenger. so that's 3 champs in theory that can go bot lane and replace a marksman. its not much but its a start.
and as for a reason to pick the adc? its just a playstyle preference to be honest. there can't be much of a strategic reason outside of pro play since really in solo queue its all up to you to carry your 4 potato teammates.
I think right now all of the adcs have a pretty unique playstyle and feel different compared to how they used to. they used to all feel the same. Caitlyn had literally no reason to pick her besides "i want to crush my lane) and did not feel unique in anyway. Corki was just your standard adc who went trinity force instead of the typical crit build. now that he's a magic damage based champ and has his passive he's way more unique. quinn had literally zero reason to play her before the marksman rework. now she has a unique split push and zoom zoom across the map playstyle.
i really don't think there's any way to make marksman have a strategic reason to pick them in solo queue as no one really picks for strategic reasons as you can't rely on your teammates. I mean are you going to pick malphite or wukong just because XxYasuoisDaddyXx locked in his main with a 43% win rate?