This is only true because 2/3 of the published sorcerers (draconic, storm) specialize on a single damage type. However, the defining characteristic or sorcerers that has been spouted at us since the playtest, their defining characteristic, is supposed to be their innate, flexible control over their spells. While I grant that, at the moment, it makes sense for most sorcs to focus on 1 elemental damage type, it doesn't change the fact that on-the-fly changing the way a spell works is supposed to be what makes them special.
However, even your lightning-based dragon sorcerer would benefit from Spell Secrets. Suddenly, burning hands, cone of cold, shatter, melf's acid arrow, scorching ray (!), all get your bonus damage. Seems nice, yeah?
If the Lore Master had to prepare the spell one way or the other, I would be completely fine with the flexibility. It's the adaptable, as-you-cast, spell manipulation that I think encroaches on sorcerer's territory.
I have seen people complaining that the elements are unbalanced, and that dragon sorcerers should have a way to change the element in the spells in order to benefit more from their elemental affinity feature.
The way I see it, each element works as something different in-game, they are not just boring colours that one can have resistance/immunity/vulnerability. Fire is the baseline spell damage. It usually (but not always) comes in big area effects. Lightning usually follows, damage is often comparable, but the area is typically more limited, making it better for targeting but worse crowd control element.
Cold may be viewed as weak because it targets con instead of dex. But not all monsters are the same, right? So there are those cases when one will want to target con (for instance, when fighting rogue-like enemies with evasion). Poison is kinda weak spell-wise, but is extremely common for monster attacks, and the damage potential is usually nasty (making up more than half of the given monster damage in many cases). Interestingly, it is the least resource-intensive for heroes to counter (protection from poison, for instance, is only 2nd level, does not cost concentration and lasts for one hour). Acid should do a lot of damage, but with the built-in inconvenience that it is not instantaneous.
The elemental cantrips even go one step further and give some elements nice riders, which I find awesome. In reality, I find ray of frost, for instance, more interesting than firebolt, as the former movement penalty may inconvenience some brutes in a much more meaningful way than the later having a slightly higher average damage, and the extra range does not come into play that often.
Having said all that, I feel like easily changing elements downplays their differences and impoverishes the whole game experience, so I definitely do not like this feature on the new wizard tradition.
Now, one may ask me, so what should be the deal with dragon sorcerers? This will go off-topic, but I guess is fair play, as I am justifying my position which points back to the topic. My answer is simply I don't think they are elemental slingers. The resistance part of elemental affinity is nice, because it does not eat up your concentration, but the bonus damage is weaksauce, extremely over-rated. It is quite relevant for cantrip-level damage, which would definitely influence cantrip choice, but on a fireball, cone of cold, chain lightning? Empower spell will easily do more for you, specially on those forsaken moments when luck seemed to have vanished.
In the end, the elements definitely
add to the flavour, but they are not
the flavour for this origin. The dragon sorcerer has better HPs, comparable to clerics, which are often seen as frontlines. They have potentially better AC than other sorcerers (mage armor is expensive for the small sorcerer spell repertoire). Sorcerers already are better at con saves. So they are somewhat tougher spellcasters. I can see that all optimizers are disappointed, but the origin itself is not specialized in a role, and non-specialization is most often sub-optimal. Yet they are fully functional, and more importantly, they are free to choose any spells they like. For those who played previous editions, with dragons not only as casters but also full of other spell-like abilities, there are plenty of other effects that can be recognized as iconic to dragon sorcery, and as such, fully thematically adequate for dragon sorcerers. Alternate form (Polymorph), spider climb, water breathing were abilities found amongst dragons, as were spells such as suggestion, dominate, sunburst, insect plague, darkness, reverse gravity, gust of wind, detect thoughts, wall of stone, just to list a few.
