I agree with your analysis but disagree with your conclusion. In my opinion (very casual/occasional gamer, have played multiple iterations of Mario Kart), the blue shell is designed to be an agent of chaos. More generally, the game itself appears to optimize for position churn and unpredictability instead of a consistency that can be gamed by skill. Under the paradigm that Mario Kart endorses, the blue shell appears to me to be very well designed and essentially accomplishes its goal.

From what I can tell the developers have always wanted to maximize the amount of chaos in order reduce the modal player's ability to consistently win. From that perspective it seems designed specifically to avoid kingmaking; the only way I've seen kingmaking work in effect is by having a confederate in second place who passively avoids aggression against first place and absorbs aggression from other places (items). The blue shell is arguably the most important item for eliminating a "first place moat", and under this interpretation it makes sense that you'd never get it in second place (which would only help second place, but no one else, and which could allow even more egregious kingmaking, because the second place user could threaten a "nuclear option" for all who pass).

Moreover, in the latest iteration the blue shell slides across the ground, which means it can harm second and third place as well. Confederates attempting to absorb items to shield first place are no longer capable of doing so without a horn. From a (literal) game theoretic perspective, the confederates no longer have an incentive to shield first place when a blue shell is coming, because they have nothing to gain (the item will still proceed) but much to lose (they will still be harmed). It's more rational to avoid it, in which case at least they can attempt to advance their own position. This further contributes to churn rather than consolidation.

So it's actually a fairly complex mechanic. It can be used to eliminate a first place moat directly, but it also undermines concentration of power in the second and third place confederates because they have no rational reason to aid first place when one is coming. And this is in addition to the fact that the blue shell can also harm the mid-race participants in fourth through n place, where n is the blue shell user. You really wouldn't want the blue shell to be used in the front-race, because it would not meaningfully disrupt the state of racers behind the user. In further contrast, it could also be misused by anyone who has possession of it as a tactic for disallowing people from passing them. If the blue shell were not given to back-race participants, there would be no way to attack first (and now even second) place when the participants are too busy fighting with each other and defending themselves to advance.

This is not to say that Mario Kart is not a game devoid of skill; you'll still see people who regularly win. But it does seem to prevent people from being able to e.g. get a good start and stay in first place through sheer inertia, by virtue of being away from the rest of the race's chaos. The chaos can be brought to any position with a blue shell, just like it can with the lightning bolt or (in the most recent iteration) the Boo items. I think it would interesting to run simulations on race results using computer players with various combinations of items to see how consistency and position churn change overall. It's my hypothesis that the game is actually more meritocratic this way, because it takes more skill to resist the entropy.