Just pick and choose who to level up. Releasing heroes is necessary to the game revenue for the developers and to prevent game stagnation for the players. So many of us have undeveloped heroes that are awaiting word if they are worth promoting or fusing. Most of the newer heroes have been good and fun to use and to compete with. Some of these newer releases allow me to compete against teams with Goram and Sylphi -- the older ones just don't do it in many cases.
I disagree with the idea that releasing heroes at the current rate is necessary to maintain revenue levels and prevent game stagnation, though admittedly we don't have access to Lilith's numbers on the matter. Soul Hunters has more value than just as a competition-based society, and while hero-based feature accretion is cheap and easy, it has a long-term cost.
The problem is that the current level of hero accretion may eventually make the game less and less approachable to new players. Churning out new heroes adds breadth but not much new depth given the current design, and new players will be faced with an ever widening swimming pool, as opposed to an awe-inspiring growing water park. This includes potential whales, who may perceive a higher cost over time to getting close to being competitive within SH, and may decide not to even start just by glancing at the guides and seeing the number of possible heroes and the large cost of maximizing their effectiveness.
In MOBA-like games, players are
features. Hero accretion favors existing whales but not new players, including whales. Thinking long-term, hero accretion may eventually harm the new player experience, which means by adding in new features now (3 heroes a month), they may be sacrificing features (new players) in the future when the number of heroes is just plain overwhelming.
I'd recommend taking a look at improving current features, like making a more inspiring campaign, giving the heroes more hero-specific challenges and personality, adding targeted challenges and special modes to the Crucible of Fire, etc etc. Competition is a great motivator for spending, but it is not the only one. There are other player types who play games like this, and Soul Hunters has decent core mechanics that can be used for so much more than making new heroes for competition whales.
Admittedly, SH is more than just grind and compete, but as I'm now at level 90 and have unlocked a lot of the features, I'm finding I'd like more game outside of the grind-and-compete class of features. Yay, a new hero comes along. Now I can grind to level it and then compete with it. So that I can grind more and compete more, and then get the next hero so that I can grind again and compete more. Wait, maybe that new hero will do slightly better in the raids, levels, etc that I have already completed! Except that I have already completed them, so the amusement from that is maybe only very slight.
Long-winded, but basically I'm crossing my fingers for Lilith to design more quality experiences for me to have within Soul Hunters, rather than just designing more heroes.