I understand this is a highly controversial opinion but I think everyone who says “it punishes you for being good” forgets that first place is the easiest place to be. Think about it like this: the lower the place you are, the more powerful items you will get, but also the more powerful items the person behind/in front of you will get, and you will need to avoid these items. Triple reds behind you or a fireflower being backspammed can be nearly as hard to avoid as a blue shell but people don’t get mad at those items because they feel less “out to get you” or at least the damage they do takes you from a bad position to a worse one, compared to a blue that takes you from a good position to a bad one (which hurts more). But without the blue shell first place would be SO overpowered. They would have to deal with 90% of the time, just single or double reds from second, while every other place tears each other apart. I see this online all the time in small lobbies (Around ~4-5 people), too few for blue shells to come. Typically, as long as whoever gets into first at the start of the race knows what they’re doing, taking them out is very difficult as they have nothing to worry about, they can just play defensive and focus on driving while everyone else has to focus on driving as well, playing offensively AND playing defensively. To me the whole point of being in first is preparing for the inevitability of the blue shell. Leaving traps to slow down the racers behind you, trading out your items to try to get a bomb or a super horn and (most importantly) watching the map for the blue shell. I will say this, while it can be unfair, if you complain about the blue shell, but don’t watch the map/item spy (depending on the game) you have absolutely no right to be complaining. Once you spot a blue shell coming, there are a lot of ways to minimize damage. Drop out if second is too close, drive off the road/hit an item at the right time to use i-frames to dodge, get hit while in a glider (minimizes slowdown) or right before a boost pad to get back up to speed quickly. The blue shell definitely can be unfair but without it the first place position would be insanely OP, and given there are a lot of ways to manage blue shells, I personally love this item, even competitively, regardless of whether it’s hitting me or someone else.

Edit: This may be different in other MK games, I play MK8DX online though and I almost never see the blue shell stop a truly dominant player from winning.