EzekielRaiden
Follower of the Way
It establishes a baseline, which is somewhat useful for reference. If anything, it might help DMs to realize that it's okay to make these sorts of decisions, and they're expected to actively make choices about what to bring in or throw out. That's a fairly significant part of the DMG, as well.
Yeah, I completely disagree. This is not the equivalent of providing a baseline and then explicitly telling the DM, "Change what you want!"
This is up-front stating that certain things are fundamentally part of the canvas, not something you paint. And that strangles creative freedom.
They use Half-Dragons instead, which are virtually the same thing. Langdedrosa Cyanwrath was the very first mini-boss you fought in the very first adventure path of 5E. And then Rezmir is another one you fight several chapters later.
If you want to split hairs that dragonborn aren't half-dragons, fine... but I'd be hard-pressed to tell you what the differences between them are.
Half-dragons have a dragon parent, and are liable to live very long lives, if they don't get themselves killed doing something stupid. Because they have a specific type of dragon as a parent, half-dragons will always resemble that type of dragon overtly, and will always have their specific ancestor's breath and resistance types. They also have tails, and sometimes wings. I *think*--but don't know for sure--that being a half-dragon provides different, color-associated stat bonuses.
Dragonborn have dragonborn parents, and have life spans essentially equivalent to a human's (though they have a much shorter "childhood" and a much longer "adolescence" than humans). Dragonborn can be nearly any color, including several colors dragons usually aren't, like brown, rust, or ochre, and their color need not have any specific relationship to their breath type or resistance. All Dragonborn have the same stat bonuses (+2 Str, +1 Cha), and no dragonborn have tails, and I'm fairly sure none have wings, either.