How many hybrid dragon-person races do we really need?
Zero.
In RPG land, the answer to the question of "How many X do we really need?" is always zero.
How many elves do we really need? Zero. Just put out some sort of race toolkit and we can build a elflike creature (whatever that means to us) if we want. How many swords do we really need? Zero. Just put out some metric for determing attack damage and we can fluff it as a sword if we want. How many monsters do we really need? Zero. Just put out a monster construction system and you can make all the kobolds and beholders you want.
So I think perhaps a more relevant question might be: how many hybrid dragon-person races do we want?
Underlying that question is: how much do we want to say about how the various hybrid dragon-people races are different and distinct? How much time and money do we want to see spent on that distinction?
In the case of draconians, it looks like dragonborn basically do the job fine (don't know much about draconians, but sure, I buy it). They don't want to spend much language in the PHB explaining how they're different, and, apparently, they don't want to spend a lot of time distinguishing them mechanically (death throws replace breath weapons, bam!). They also likely wanted to demonstrate the power of the re-skin with that.
In the case of, say, half-dragons, it looks like WotC figured they're different enough to warrant a separate entry.
This isn't to pick on the person who posted this, really, it's just a pet peeve of mine. This reductionist "we don't need different kinds of X!" philosophy is baldly true, but then often in the world the same person who espouses that will turn around and buy a book of 100+ different monster stats that they don't really need, or 20 or so different character options that they don't really need, or whatever. Because in practice, the difference can be valuable.
Perhaps in the draognborn/draconian field, it's not so valuable as to bother worrying about for most players. But in other places (say, the PS tiefling/Turathi tiefling distinction, or the meat HP/morale HP distinction), it can be valuable, at least for some players. And the people insisting on those distinctions shouldn't just be written off with "Do we
really need that?" because of course we don't, but that doesn't mean it's not something we really want.
ANYWAY, back to your regularly scheduled thread. Since I'm not big into DL, I don't know if the draconians-are-dragonborn looses anything in translation, but if it doesn't, yaaay!