We can argue about whether Atomos was awful, but I think it is fair to say that the rebalance made him better:
If he is your only five-star green and he has been for “some time,” is it likely that you’re going to get another, better five-star green soon? You’re likely C2P or F2P (props to you, for that!) if you haven’t scored another legendary green. So, you could get a five-star green from TC20, but will that hero be markedly better than Atomos?
It may also be useful to consider Atomos in the context of the rest of your roster. Do you have a good tank? I see Atomos as a tank, so if you have a tank you’re happy with, he may be less desirable.
On the other hand, if your alliance coordinates tanks and you need a green tank, he may be a good fit.
Do you have other Atlantis family heroes (like Ursena and Poseidon)? The family bonus of the Atlantis family gives them a defense boost. If you have other heroes from that family in your defense team, then he would be a good addition to your defense team.
There are several threads around heroes in whom folks regret investing materials, and Atomos was on a few of those lists. I think he’s likely off those lists with the rebalance. But, even if he is a “B” hero, he may still be a good fit for you. Horghall was my first (and for a long time, only) five-star green. I leveled him up, loved him, gave him emblems, and he got me through a lot of the map. He is no longer my favorite green, not even my favorite S1 green, but I don’t regret leveling him up because he filled a need for me at the time.
Good luck.