It's already more work for the developers because it's a complete waste of their time, money, and effort.
It serves no tangible purpose beyond fan rules that don't exist whatsoever for the development team. The only reason for them to care(and throw away the work done just because of a silly fan rule) is Sakurai is actually making it the case. But he knows as well that it doesn't really do much at all. You click "disable it under specific condition" and move on. That's literally the brunt of that work.
Beyond that, let's note Spring Man's AT stuff. He has a full out jump animation, his own getting hurt animation, a rapid jab, and a full punch animation, plus a taunt no less. Now how many Stage Cameos(not hazards) would contain all of this? Especially when none of these would affect the players, meaning no hitboxes or hurtboxes. So it's not the same amount of work. A hazard, yes. But at that point, if Springtron isn't an alt, why not just make him a Hazard and not throw away the previous data as is? Spring Man could still appear on that stage too as an AT as long as Spring Man(assuming he's part of the ARMS character, if an alt situation), and Springtron can be his own thing without creating any issues or spending significantly extra time nor wasting data for the sole sake of a fan rule.
What benefit would replacing the AT even entail? What, a fan rule? It's not rocket science that Sakurai isn't gong to waste time on worrying about that kind of thing. Now what benefit is there to keep the AT existing? It means AT's aren't a death sentence. It means polls that happen will have people ignore these silly fan rules and be as honest as possible. It means when he gets data, people will have the best information and ideas they can muster. It only helps the game. Reality is, fan rules have done nothing but made speculation go downhill for ages on end. It kills hopes, it creates a toxic atmosphere(that's not to say they aren't useful information either. It also means people can think up new roster ideas they wouldn't normally due to a character they think are "not happening"). However, there's no denying the atmosphere is completely unfriendly due to fan rules. There's a huge difference between a game with roster rules and fan rules hurting speculation.
So I ask you; what's the real benefit of basically making a stage hazard to replace a perfectly working AT, while redoing every animation as well. And if you aren't sure how hard this can be, it's common knowledge that a lot of Model Replacement Mods are actually not remotely easy and people take a long time to iron out all the animation kinks. Now, yes, they are professionals with Sakurai's team. Sure. But it also took a long time to make Meta Ridley work, so it's already proven this isn't an easy task, even for them. New model changes are not remotely easy, no matter how professional you are. Replacing a model with another one takes a lot of time and energy and effort. Time they could be spending on making actual new material, not a very pointless AT change(and it's pretty pointless as it has practically no benefit to anyone but maybe some speculators).