Well......Aurelion kinda doesn't do much outside his stun? I mean the moment he´s used it he isn't as interesting anymore.
Most mages are like that, but because Aurelion only really 1 skillshot aside from his ulti it grows more noticeable-
Like, most people probably find watching him float around trying to position his stars about as interesting as the marksman right clicking , then you add the fact that a lot of stuff happens around the marksmen and suddenly they are more interesting.
As for the reason you lose fights if the marksman dies.....no other champ can kill enemy tanks anymore except a very rare few.
Send Aurelion to kill enemy
with
? It will take "ages" and it wouldn't be suprising if cho killed aurelion first with E+ auto attacks.....
Nearly the same on maokai, seju, zac and a few other tanks, send mage to kill them? GG mage will lose the 1v1 because they don't really cope well with the new tanks and their items.
Send marksman to kill tank? Depending on which one the time will vary but generally if the marksman has built properly ( AD itemization superiority galore) then most tanks will die within 20 seconds.
Marksmen focused or better suited at dealing with tanks might do it in under 10-15.
will do it in 7.
TL;DR
Mages aren't as interesting when they have dropped their combo, and they really suck at dealing with the updated tanks and their items.
Chances are the tank will kill the mage long before the mage get's anywhere, and that's why it's really bad if the marksman dies since that's the only champ capable of handling the tanks.
Tanks have been balanced to be weaker against sustained AD damage, and AD itemization is vastly superior when it comes to handling tanks too.
Thus one could say that half the reason marksmen are so important is because the tanks are so good and mages so mediocre at handling them.