I would suggest there are plenty of reasons why the half-dragon would want this duel, and then let the PC live at the end of it. However, my guess would be that what I would consider perfectly good reasons you would disagree with (seeing as how you probably know the scenario and have thought about it, and have thus far not come upon any that you have found valid.)
I find a lot of rationalization about published material by gamers on the forums. If it is in print, then it must be good and here are the reasons a), b) and c). Not that you are necessarily doing this, but I do have to ask if you are trying to be totally objective in your rationalization of this duel as a good encounter.
Is practically forcing a PC to take a dirt nap REALLY a good encounter idea? Or is it just an encounter idea that happens to be printed, and hence, the DM should just go with it? Does it really add anything to the game, or is it just a heavy handed way to introduce a reoccurring villain?
Suffice it to say... you and KarinsDad don't like the scene and/or the adventure. That's cool. It ain't gonna be to everyone's taste.
Agreed. But like I said in my last post, this adventure feels unpolished.
One of the things that appears to be happening is quite a bit of railroading. I always thought that for major published adventures like this, the authors should use a flowchart where there were multiple entrances and exits from each interaction.
The Lost Mines of Phandelver book seemed to do a fairly decent job of this type of thing with the town of Phandalin. The PCs had a lot of different NPCs to interact with, each of whom had their own goals and agendas. There was no set order in which the PCs had to approach the NPCs. They could do some missions, avoid others, and not even know of some.
But so far, HotDQ has appeared to be:
1) Go to a town where a dragon is flying overhead (WT?).
2) Have first easy encounter.
3) Move to second easy encounter (as directed by NPCs).
4) Go to keep (as directed by NPCs).
5) Talk to governor.
6) Go down tunnel (as directed by NPCs).
7) ...
12) Face off against a blue dragon (WT?)
13) Be forced into a lopsided duel with a half dragon (WT?)
Each encounter appears to be linearly following the previous encounter with virtually no ability to really go off the beaten path. I don't know if this is really true, or whether that is just my perception based on our game.
It just feels like a subpar adventure so far. To me. Obviously, YMMV.
And don't get me wrong. I am having a lot of fun with it. Playing with my friends and having a blast. That's one of the things about D&D. Even in a subpar adventure, the players can still have a lot of fun. I just feel that WotC missed the boat on having a great 5E flagship adventure and settled for a so so one instead. Still usable and still fun, but lacking in greatness in so many ways. Nothing in it so far that made me go "Wow, that was cool"

, but several things in it so far that made me go "Wow, that was pretty lame"

.
And trust me. I would SO much more want to say "Wow, that was cool". I might be critiquing this pretty harshly, but it's just because it has been somewhat disappointing compared to the quality of the 5E PHB, for example. Even Lost Mines of Phandelver with all of its first real adventure warts seems like a much better product.