No. We know from half-elf, the offspring is an entirely new race that isn't the race of either parent. We also know this from half-celestial, half-dragon, half-demon, etc. from prior editions.
We know no such thing. Firstly, you haven't defined race, secondly you haven't shown how something like a half elf is a new race, yet a sea elf is a subrace. They are listed seperately, I can give you that much, but that doesn't mean they are entirely new and unrelated races.
Not really. In 5e they seem to be focusing on all the "planetouched" being descended from other planar beings and humans. Not sure why they keep picking human over and over as elves, gnomes, dwarves, halflings, etc. can all make half-whatevers and then have descendants.
That doesn't really matter, though. A demon is not the same race as a devil who are both not the same race as celestials, etc. The various bloodlines, despite all being linked to outer planar creatures, are going to be different races as we have seen in 5e, as well as 2e and 3e.
None of this disputes my claim. In fact, by saying that fiends and celestials aren't related (which again ignores that some devils at least are fallen angels, making them the say "race") and making that your full point, you reinforce that you are simply ignoring the other half of the people under discussion.
As for why they keep picking human, we literally discussed this earlier in this thread. It would be because of the difficulty of balancing abilities of elves and dwarves in addition to the abilities of tieflings and aasimar.
Let's assume you're right. They're still all different races, as the founders of those bloodlines are different races(devil, demon, demodand, etc.) and the half offspring are not the race of either parent.
And your second assumption in this (that half- offpsring are not the races of either parent) is not proven, Because again, you have no defined race in anyway. Why does having a human father make you a completely new and unseen thing, but being able to breathe underwater with gills and fins... still makes you an elf?
No it's not as it hasn't even been done yet. Taking a race with subraces and re-making them in a new way, doesn't mean that they are going to in the future take multiple different races and merge them together into a unified race.
It doesn't mean they haven't either. My enitre point is that it seems immently possible, since it appears to have been done.
In D&D 5e it's anything that isn't explicitly a subrace.
Okay, a definition. So, explain to me why zombies are a race then, since they are not explicitly a subrace, by your definition, they are a new race.
Because that's the way D&D 5e presents them. It presents elf as a race, and wood elf, sea elf, etc. as subraces. It presents giants as a type, so the various races of giants are merely the giant type, not one unified race.
Humanoid is another type. In that type are humans, elves, halflings, orcs, etc. All of those races have similar biology, but are not one race.
If you want to know why they did it, you need to ask Crawford.
So, it is completely arbitrary, follows no rules, and is therefore quite meaningless.
Because they are not explicitly subraces. All of them are presented as separate races. Genasi, aasimar and tiefling even have their own subraces. Tiefling is the race. It has base race traits. It has 9 subraces. One for each planar ruler of Hell. Genasi is a race. Air genasi is a subrace of genasi. Aasimar is a race. Fallen Aasimar is a subrace.
This is basic stuff man. In 5e they are not some unified race called planetouched. At best you can assume that there is a type called planetouched, and in that type are different races called genasi, aasimar and tieflings.
I'll remind you of the dragonborn again. Having subraces within subraces doesn't disqualify anything. Additionally, Ghosts and zombies are not subraces, so therefore by your argumentation they are new races of beings. A lycanthrope isn't a subrace, therefore they are also a new race. This is your present argument, and I find it utterly lacking.
You need to look up what descendant means. It doesn't mean a child. It means that you are descended from an ancestor. Not parent. Ancestor. The Cambridge dictionary lists ancestor as meaning someone who you are related to who lived a long time ago.
No they are not. That's why we have the world "children" and use it, not descendants.
Yes words work a certain way. Just not the way you are trying to use this word.
As posted by UngeheuerLich immediately under this.. you are wrong. Descendants include your children. Ancestors include your parents.