I hate the argument that if they create a sandbox mode there won't be enough player engagement to go around because it offers no rewards. If that's the case why are people asking for it? The whole point of collecting characters is to use these characters in the game. The problem is this game has become so top heavy that many characters honestly can't be used or there's limited places to try new combinations of characters and actually gain that knowledge of how it performed.
Also, how many games have modes that are not reward driven? There's tons of them. Halo's Forge mode is a sandbox with no rewards. People loved it. Nazi Zombies had no rewards (at first) people loved it. Nazi Zombies wasn't even supposed to be in the game yet people loved it and spawned a whole new game mode for years to come. I get that those games weren't free downloadable phone games but this game has also made a billion dollars so money shouldn't be an issue.
The problem is, for CG it is. They stated clearly if it's not something they can do for free they will NOT do it. Every other reason they state doesn't even matter cause the underlying issue is $$$. If a company straight up says they won't do it because it's not free to do, clearly they're only driven on $$$ and not the player base. It's as simple as that.