As much as I like Madara, I have to ask: What would be the point in keeping him around?
Madara is quite a different case from both Frieza and Buu. Frieza was pure evil from day one and he is evil today. He was brought back in Resurrection F (and associated DBS saga) because he made for a powerful adversary to contest Goku and Vegeta's (at the time) newly acquired Super Saiyan Blue form. Plus, Frieza actually had an in universe reason (that actually related to his character) as to why it would be good for him to come back. Even if it was just for the sake of pure revenge on Goku, that is still actually a legit reason the (that fits Frieza's character) as to why the writers would bring him back.
Then, the tournament of power was also the perfect setup for Frieza to come back again. Buu fell asleep, and universe 7 urgently needed a strong last team member. That is why, for the fate of the universe, they decided to call a temporary truce with the ultimate evil! It was absolutely necessary, and this return of Frieza actually perfectly made sense with what was going on in the plot.
Buu is a bit of a different case, but it still makes sense. The fat Buu becoming good was actually apart of the plot of that arc, and he ultimately helped in and survived the fight with Kid Buu, so it makes sense that he just chills at Hercule's place afterwards. The writers didn't even use him that much afterwards, but the reason why he is a remains still makes sense.
Madara is different though. Upon Black Zetsu's betrayal, Madara realized that all his effort was lost and for naught, and upon his death, he actually came to accept Hashirama's viewpoint. At that point, everything that made him an effective antagonist pretty much got resolved and disappeared. If he were to be edo'd or revived again, he most likely wouldn't actually be making any big revolutionary moves anymore (like the eye of the moon plan), or attacking Konoha. Furthermore, he didn't split into any "Good Madara" and "Bad Madara" like Buu did, so there was no reason for a part of him to survive or anything like that.
Basically, Madara's story and overall relevance ended when he died at the end of the war. There's no revenge plot with Madara like there was with Frieza (since Madara died in peace), and thus far there hasn't been any situation like the tournament of power that would actually require our heroes to consult the dead and old enemies in order to deal with some grave situation.
Even with Salamander's "Madara clone" plot, I still don't get what the point of cloning him would be. Who would clone him and why? Like, even Orochimaru had his own reasons for creating Mitsuki, but this Madara cloning just sounds random (especially if the scientist or whoever clones him just lets the kid Madara go live in Konoha). Salamander once said that the purpose would be harvesting a rinnegan, but then what would even be the point of that? What are they using this rinnegan for? Madara (the original) sought to awaken the rinnegan because it was actually necessary for summoning and controlling the mazou and carrying out the eye of the moon plan. Obito later sought to obtain it from Nagato for essentially the same reason. In other words, their pursuit of the rinnegan had an actual purpose at the time. What would be this random scientist's purpose though?
Also, even if he had a purpose for the rinnegan, why go through the trouble of cloning Madara and sending him to Konoha in hopes that he eventually awakens a rinnegan for you to subsequently take, when you can either just seek a chakara fruit to awaken one, or steal Sasuke's?
I know you may say that it would be too hard to steal Sasuke's, but if this clone Madara gets to the point where he actually awakens the rinnegan, then it would most likely be just as hard to steal it from him as well! Rinnegan users aren't just some pushovers whose eyes you can just casually steal like taking candy from a baby. Therefore, if you're going to try and fight a rinnegan Madara, you might as well just save yourself the cloning effort and just go for Sasuke's since his already exists, and either way, you're in for an intense battle.
Really, the chakara fruit idea would be best if you want a rinnegan, either that, or find Rikudou Madara's lower half, and instead of cloning Madara with it, find a way to extract the rikudou chakara from it either into your own body, or into some kind of drug, so that you can just awaken the rinnegan for yourself instead of trying to somehow survive a fight with a rinnegan Madara.