I think you are looking at this from the wrong angle.
Yes, on a meta level the author did give him the plague scales. But on a narrative level it is something so much better. It actually creates stakes in dragon evolution choices. Where instead of boring generic isekai with system with no drawbacks, he has to make hard choices to become more powerful and protect others from threats, despite it having hard consequences(like poisoning human friends who stick around too long).
Irushia made the hard choice, choosing an evolution that would get him ostracized by necessity, in order to gain the power to protect his friends and comrades and survive, rather than the evolution he would've wanted(noble dragon form). He made that choice without guarantee, but with hope that he could continue to evolve until one day he could evolve away from such a disastrous type of dragon into a more heroic/non deadly one that can live freely with others.
Him making that choice is what makes him heroic.
This isn't just some "I was reincarnated as a dragon, so I will live my life peacefully in a backwater village with my human friends and monster buddies as king of the mountain" series. Nor was it ever meant to be, nor was it building to. It's building up more than that.
I really can't say much without spoiling stuff directly. Suffice to say you are giving the author WAY less credit than they deserve. You are looking at the short term, when the author is making a much larger story.
(Which I don't blame you, this manga progresses very slowly with its release rate(not the fault of translators), which makes everything feel way more dragged out)
We know factually that characters can acquire skills from repetitive exposure/ practice, why can't something like that happen for companions over time, or why can't irushia gain companions immune to his scales?
(A rhetorical question, I am not implying any of that does or does not happen)
Did he though? Did he really lose those connections, or are they just separated for a period of time before possibly reconnecting in the future? How might those characters react and make choices to Irushia going missing? What would irushia do if he could neutralize his ability and could learn where he had been born(and thus find his way back)? What would he do in the meantime?
It's like saying because the strawhat pirates were separated at Saobody, or because the straw hats moved on from Alabasta, that everything was just reset with Vivi. Or that goku was reset because there were some stronger fighters in the world/universe that he meets and finds may be stronger than him at first.
You may still decide this story isn't for you, that you wanted "slow life after being reincarnated as a dragon" or whatever you had in mind. My point here is just that some of your perspectives seem to be way off, in a way completely unfair for any series. Like this
Where you missed the entire point. Not all abilities are going to have "benefits" one would use. One Piece has devil fruits give all their users the ability to become completely weak in water. It's a package deal, for the other powers that come with it. If I became a mutant where my body is covered in porcupine/tarantula spikes, it's a "stupid power" that keeps me from being able to embrace or touch other people. Same if the ability made my skin excrete toxic mucus like a poison frog.
Or beast or mystique in x men, getting stuck with blue/scaley/hairy bodies for their abilities to teleport or shapeshift.
Plague scales, absolutely can be useful. The ability to passively poison enemies and armies just by flying around would be devastating for any enemy that had to confront irushia and could not finish him off quickly. And it should becomes even more useful when leveled up and it becomes more fast acting. It's why the nearby city wants Irushia hunted, because it is a dangerous power.
The problem is it is not a power irushia wants to have or use. It's a power he accepted in the bundle of powers for this evolution. One that limits his interaction with others currently.
Irushia is a frog in the well, a big fish in a little pond. One who continually is reminded of that fact, but who manages to make progress and become something great due to his tenacity, creativity, and by choosing the harder more uncertain path.
