And, again, what is the significance of this? That it's fine for my friend to not get stuff he can easily use going forward, because he previously got stuff he could easily use? How do you figure?
I know this is likely to get me "Aha'd" as revealing that I truly am a heartless individual, but it really is quite simple. Gonna call your friend Bob just to save typing.
Bob has been on the rollercoaster over 30 times. There are, counting races and subraces, over a hundred options for Bob to choose from. And yet, at the idea that we might get something under our preference, that we might get a turn on the rollercoaster, we suddenly start getting "But what about Bob, what about his preferences? Shouldn't we make sure Bob can keep riding?"
And yeah, I'm a softie, I want Bob to keep having fun, but I also recognize that he has had fun. He has over a hundred options to pick from under his preferred system. Is it truly so cruel for us to want... 10? I mean, three lineages being playtested, with a note of "this is our future design direction" suddenly got you banging on our doors, demanding to know what we are going to do about poor Bob with his hundreds of options and 7 years of having things his way. And, I hope he gets his sidebar, I really do, but is it really so terrible for us to ask for a turn?
Fair enough, I suppose that was rude.
That Wizards already acknowledged draconians as a type of dragonborn, and kender as a type of halfling, doesn't mean they wouldn't get a writeup in a hypothetical Dragonlance book. Why wouldn't they? There are Dragonlance fans who would like to have them, and they'd be part of the market for a Dragonlance setting book. Note that along similar lines, they actually took a stab at grugachs in UA, even though the PHB said they were a type of wood elf.
It depends on what you mean by a "write-up". In Wayfinder's Guide to Eberron the Aerenal and Valenar elves got their lore written up, but their mechanics were just a sidebar of variant features. And when they went to Rising from the Last War, they didn't even do that much, just giving us the Lore and no new mechanics.
So, will they get a "write up" that includes their lore and a sidebar variant feature? Yes, I think so. Will they get a write-up that fundamentally changes the race with new ASI's, language, abilities, ect... I don't see why they would. They haven't done it before, really, every setting has been pretty close to the baseline. With the exception of Wildemount, which was written by Matt Mercer for his own campaign needs, not by WoTC.
Also, note that the Grugach happened in 2017, before Mordenkainen's and offered us Sea Elves and Shadar-Kai as well. I think it is clear that design was for that book, and considering the changes since 2017, and the fact that the Grugach didn't make it in, that things which are too similiar are likely not to. Though WoTC has a bad habit already, since we have Tritons and Sea Elves, for some reason.
As for all the others, "similar races exist and you can modify them" is not a reason for Wizards to pass on a writeup. You could make that argument about any of the new races that appeared after the PHB; orcs could just be modified half-orcs, for example. (See also my previous argument about how "homebrew exists" is not compelling.)
Again though, it depends on what you mean by "write up". To give another example, look towards Goblins. Goblins are vastly vastly different between Ravnica, Eberron and Faerun. And they gave them "write-up" in all three books... which are just copy paste in terms of the mechanics. Literally identical. From the mad bomb boys to the sniveling cowards to the proud heirs of an ancient kingdom, they were treated identically mechanically.
So, would you really get different mechanical write-ups for the Kithkin? Would it really be more than just a single sidebar variant feature? What kind of "write-up" are we talking about?
In any case, note we agreed that modrons and thri-kreen are distinct enough to merit their own writeup; you also raised spirit folk as another. Thri-kreen are practically guaranteed if we get a Dark Sun book, and modrons are likely if we get a planar/Planescape book (due to modrons getting attention in core 5E, and Nordom from Planescape: Torment, a game which the designers are clearly fond of).
Again though, it depends on how they approach it. I can easily see Spiritfolk and Kreen being designed like lineages. They encompass such a broad category that it simply makes sense to try and do it that way.
The only one that would be hard is the Modron. A "planar entity" lineage wouldn't really work, the various planar entities are too distinct. But also, Modrons are just really hard to play or make players. You can only really play the crazy ones, and "crazy construct" is covered by Reborn in some ways.
I'm not sure where it would get pushed, but considering that is the only race I can't see falling easily under this new design umbrella, and I think it would be a very difficult sell for player characters anyways... I think it is more likely that we aren't getting Modrons.
If you stop coming up with reasons why we don't need official defaults, I'll be happy to stop defending the idea. But as someone who endorses the sidebar, I'm obliged to respond to criticisms. Nothing personal.
Sure, nothing personal. Just seems that I've repeated my stance quite a lot to keep getting asked the same qestions