Guardinals were a thing in 2E. Straight out of Elysium in the outer planes. But they haven't really been a thing since then. So it's no wonder why people were sniffy towards Aardlings, which were meant to be prime plane descendants of Guardinals originally. If Guardinals were more prevalent in the game... having a player race for them would have been more acceptable as something the game could "want".
Do I think there needs to be a "fiendish" version of Aardlings? No. Because there isn't a "fiendish" version of Guardinals. Instead, the racial mirror equivalent would be a player race based upon daemons/yugoloths, as Hades is the opposite side of the Great Wheel from Elysium.
Now that being said... it also is going to come down to exactly how the designers wish to define which planes Aasimar and Tieflings are descended from. They've already made "demonic" tieflings, just like the standard ones were "devilish". So they are saying that the descendants of the Lawful Evil outer plane and Chaotic Evil outer plane both fall under the category of "tiefling". Which should mean really, that decsendants of the Neutral Evil outer plane (Hades) should probaby be considered tieflings too.
Which means on the Celestial side... Aasimar should thus be the category of the Lawful Good archon descendants of Mount Celestia, the Neutral Good guardinal descendants of Elysium, and the Chaotic Good whatever-creatures-come-from-Ysgard-anyone-really-know? descendants of Ysgard are. They all should be considered Aasimar.
And the two remaining outer planes of Lawful Neutral Mechanus, and Chaotic Neutral Limbo? Maybe the Modrons and Slaad get their own special descendant races, or perhaps we just suggest that those two planes just couldn't support or ever have relations with Prime creatures to actually create descendants of those planes?