Marksmen are not fit to play any other lane (except a few rule exceptions). If they would, it means they will be game breaking in the current environment. So they need to stick with a sup in the early game, even later on depending on the team comp.
A lot of players are playing marksmen and making them obsolete means throwing a big chunk of the fan base.
Also, marksmen where in the duo lane since at least season 2 so I don't know why people try to force marksmen out of the bot lane. This game needs more AP marksmen, but that's another story and is more about the actual lack of items and champions
You can also play atm: Ziggs, Irelia, Ahri, Akali, Azir, etc bot with success. But you need a decent sup pref with healing