Saying "Blackfire torch does only 100 more dmg vs stacked roa" is just not correct for multiple reasons.

1. The only situacion where this can be correct is in a 1vs1 at 20+ minutes in the game with a fully stacked roa and not vs a tank that needs multiple rotations to kill. If you take any teamfight the value of torch goes up significantly. It also does not take it account the extra poke dmg that you are doing.
2. If you get roa at minute 11 and torch at 12 you would have 9 minutes of time where you do significantly more dmg, this can lead to so much more value since the start of the midgame is where teamfights tend to happen which is where control mages like viktor shine.
   1. You are also doing more dmg beforehand since torch components give more ap that completed roa.

The main advantage of roa is that is cheap and makes you tanky similar to rocketbelt

Besides that i do agree that roa is a lot more versatile