I understand your argument, and in a hypothetical situation where you could build a team that could keep him alive infinitely then yes the skill would eventually become really powerful. The problem is that you need to be able to place the fear for that to happen, and block debuffs and full team cleanse has become nearly ubiquitous for support champions.
I just took a quick look at the multipliers for the most popular defensive arena nukers in the game (Staltus, Ragash, Soulless, Harima) and most of their aoe multipliers are significantly higher than the base 3x that Fortus has on his secret skill. This gap only increases because all of these champions have higher base defense than Fortus does. Soulless has the highest multiplier at 4.9 on his A3, which also increases the multiplier by 10% per debuff on the target. His scaling may not be infinite but it's far more practical, and less conditional than Fortus'. You would need to place 20 fears to get him to approximately the same multiplier as Soulless has innately without even accounting for Soulless' A3 scaling. Yes Fortus' A3 ignores 30% of defense for champs under fear effects, but if you're building him in a defensive comp like you suggested you're not likely running him in Savage or Lethal.
These problems all exist without accounting for the current arena meta where placing debuffs is very punishing because of sheep. The sheep nerf made it resistable sub-6* awakening, but you need to run resist on your champ to do that which again further diminishes his damage. In high gold 5, platinum arena, or gold live arena his skillset is much more of a liability than a help. It's not practical, and you're far more likely to get killed by any number of more powerful champions, or get sheeped because you're putting out so many fears whether you're in resist gear or not because of the debuff volume.
Beyond that the A3 is a secret skill so it needs to proc to be usable. Compare that to the secret skills for Valkanen and Mortu and there's a stark contrast in power. Mortu and Valaknen both place block revive and hit incredibly hard without having to go several rounds to see a significant return from the A3. Fortus might hit that way if the fight lasted 10+ minutes, the team wasn't running any of the most popular arena supports (Siphi, Duchess, Pythion, Elva, etc.) who all bring block debuffs and several bring cleanses, and he wasn't sheeped to death while the other team picked off your supports.
So we've got a champion who has admittedly a compelling aspect to his kit (the infinite scaling) but requires so much setup and circumstance to be effective that the scaling becomes irrelevant. I'm not sure what level of arena you're playing in but it's not realistic to think you can just run him forever and let the damage build up when you're playing against endgame accounts, let alone whales and krakens.
Fortus is almost universally viewed as one of the worst void legendaries in the game. I'm not sure why you've picked this hill to die on when the community consensus is so strongly opposed to the opinion you're pushing.