Two things about this - first one being that it's abundantly clear Nintendo doesn't give a **** about how their business practices look LOL. They've been scummy as hell right in front of our eyes more times than I can count. If they feel that Tails is a profitable choice for a playable character I can't imagine them hesitating to sell him again.
The other one is that I honestly don't think it's all that scummy in the first place. Let's put it into perspective - Mii Costumes cost a little bit over one dollar. Tails came out nearly two years ago now. How many people are going to make a scene about losing a dollar two years ago when it's more likely they'll just be celebrating that a character they like made it into Smash? If anything, I think the illusion of playing as your favorite character for two years is worth the price of admission, and would make the actual reveal that much more exciting. And this is splitting hairs a bit, but I feel like it's worth mentioning that Smash 4 even sold costumes of characters who are already present in the game like Samus, Captain Falcon and Fox.
I think selling say, Travis or Geno as a full playable fighter would be pretty "scummy", since I think it's at least safe to say a character's Mii Costume wouldn't appear in the same pass as them, but since Tails was a full pass ago and literally the first wave of nearly ten now I don't think it would be very much of an issue. And as others stated, Tails' costume was probably one of the easiest to bring back since they ported it over from Smash 4.
Honestly, the more I think about it, I think Rex may have been decidedly scummier since he was used as an incentive to buy a full pass worth of content. And we know very well that Rex was planned to be a part of Pyra's moveset initially. It's just one of those things that I feel we've all kinda overthought. I do think most Mii Costumes are disconfirmed, but with Tails' costume just having been so long ago and being a reused asset as is, I think there's a lot of wiggle room.
Of course I don't blame you for thinking the way you do either - that's just what we had all accepted as the situation a while back, and outside of Rex who is a special case to begin with we haven't seen much to contradict it. But I think time has opened the opportunity up again for this particular character.
First, The Mii Costume is only 75 cents, so it doesn't even cost a Dollar
Second, This has been something I've been trying to argue on the Papagenos discord. I will say a Big argument I have heard over the Sonic Miis is that if FP2 was planned out along with FP1, why were the Sonic Miis even added in the first place
The thing is that if FP2 was a sure thing, I heavily doubt we get ARMS and Xenoblade Chronicles 2 Content, Which included Miis and Spirits, Xenoblade Chronicles 2 is even more egregious because it came with music.
I do actually think the FP2 was perhaps in consideration during the time of FP1 Finalization, and there have been some coincidences that lead to that theory, but I don't think there it was greenlit at the same time FP1, and there is a difference between Planning and being greenlit
In the case of another Sonic character, since it's been speculated and sort of agreed that FP2 was greenlit around June of 2019, after Joker's pack came out, There's a great reason to believe they put in the Sonic content in the pass in the case a full FP2 wasn't decided on. Pretty much it came before the greenlit
I actually think Timing is an issue the Speculation community has an issue with. I think alot of people argue for certain things with no consideration for how Timing is factored in, as well as other things like difference between plans and being greenlit, how Negotiations can affect things, etc.
Now I know there is also the Speculation that Steve and Sephiroth were always gonna be playable. I actually am behind this idea, but this idea could go against the idea of a Sonic character if you look at it a certain way. What I will say is that Regarding Steve and Sephiroth, since they weren't in the game in an form, if Nintendo was already far in negotiations with them, it would be a waste to just drop the talks right there, unlike with Sonic which already had content represented in Base game, since of course, the absence of things like Sonic Spirits and Assist trophies, when they were in previous games, would be very weird to say the least, especially if DLC wasn't factored in until January of 2018, when most of the base game content was decided. Sephiroth and Steve weren't in Previous games so the absence of them wouldn't be out of place
If FP2 Intially wasn't a thing, or wasn't deemed as worth the effort, they still might want Sephiroth and Steve to at least have a presence in the game, perhaps as Fighters, just to make sure the talks don't go to waste. After all, if you have the oppertunity to include Sephiroth and Steve in someway, you don't waste it. Sonic wasn't in this position since we already do have the characters represented in some way, so it wouldn't be a waste
Since we have a Full FP2 to play with, if they did think any of the Main 4 options were characters they feel would make a great add to the Playable option, now is there chance to start the Re-Negotation. Even if Planned from when DLC was first decided around January of 2018, if there happens not to be a full FP2, even if Another Sonic character was in Plans, it's no waste since at least they are in the game
As for Rex, I got to be honest, I think Sakurai knows that the blades are more popular and the more important things about Xenoblade Chronicles 2. He did add Nia's Costume to base game, so he didn't put her costume in Ransom. Not to say he doesn't like Rex (though he's never really said that, even when praising XB2), and he definitly treat him with respect in Pyra/Mythra's Trailer and moveset. I guess though it was like, if the Blades at least have a major presence, whether through base game content or Future DLC, then Rex was seen as more expendable
And for the Smash 4 costume point. I think the scenraio regarding Falcon, Fox, Samus is different from a Case like Tails, and especially Travis and Geno. When they sold the Former costumes, people already know that Falcon, Fox, Samus are playable. Going into the latter costumes, they might buy these costumes precisely because they might think this is the best they are gonna get. I will say we've never gotten a character who was introduced into the game as a Costume, then became playable later on
Not to say that wouldn't happen, and yeah I don't think people are gonna be too mad about a costume from way back then like Tails costume. Travis and Genos are stronger cases because their costumes came so recently though