First of all, I would like to thank Enixus for bringing up this idea.
The idea is having Old Event Heroes be available somewhere and I think it would be nice for it to be added to the Hero pool on hero summon rolls after the event (To make the RNG of getting good heroes higher *cough*).
Battle Girl Highschool, Chain Chronicles, Idolm M@ster CG and LoveLive SIF (Some famous gacha games) are doing it, so I don't see any flaw this idea.
This should totally happen!
Also, if they don't add new Heroes to the hero pool regularly (like event heroes), their sales will drop significantly on hero summon rolls and monster inventory expansion due to players realizing that they already have like every hero or always getting training partners when they roll.
"HEY! LIMIT BREAK IS A THING, YOU KNOW?"
Yes, but it's more of a privilege than a main feature in my opinion.
With the current Hero pool, players are getting other jobs of the same hero they already have instead of limit breaking.
With the regular update of the Hero Pool, not only that you're expanding your pool with no extra cost(since you're using old event heroes as the 'New' Hero), you'll also make other player who wasn't able to get the chance to have the event heroes they want and somehow increase the chance of getting a good hero.
More regular expansion, more gems.
More gems, more income.
More income, more salary and investment.
More salary and investment, more expansion. *Cough*
If the Moderators are reading this, I suggest looking up to the gacha games I mentioned above.
If this sounds offensive, I'm sorry. I'm just trying to be more convincing.