what if Mages go bot with the support and Marksmen go mid?

the meta is what it is because it's the best way to play the game to give your team the best chance of winning. People didn't just randomly decide one day to have a bruiser top, a mage mid, a support and adc bot, and whatever role was missing in the jungle. Literally every combination was tried back in s1, and the current meta is what people found to work the best.

Ziggs going bot doesn't mean every mage can go bot and be as successful. ADCs are still good at sieging towers and are still good bot, just arguably not as good as ziggs at the moment.

True, they originally went bot because mages needed the short lane and bot gave adc enough protection via supports. But everyone and their mother now has dashes/2000000 range ults.

Marksmen go into the duo lane because they are the least XP dependent in their efficiency.

Mages are not XP-independent.

Ziggs is the only mage bot laner currently, as he abuses First Brick to hell and back.