People seem to immediately dismiss kits unless they include splatbomb/suction bomb or cooler/trizooka/boyaah.

First, I don't think people understand that strength is relative. If every weapon got their "perfect kit", that doesn't mean that every weapon would suddenly be "good enough" for competitive play. If every weapon was stronger, the bar for being competitive would be raised ("when everyone's super... no one will be" ~Syndrome). The reality is, no matter how kits are designed, at the end of the day there will be some variation in the strength of each kit, and competitive will only play the top 20%. (it doesn't matter how strong the bottom 80% is if the there is still 20% that are better then them.

Second, weapons get re-balanced. The specials that are good today aren't set in stone. The meta of today isn't set in stone. Something that sort of demonstrates this is this post about the release of Inkbrush Nouveau in Splatoon 2: https://www.reddit.com/r/splatoon/comments/7c003v/inkbrush_nouveau_coming_nov_10_at_7_pm_pt_sub_ink/?utm_source=share&utm_medium=web2x&context=3. The majority of comments there are saying the kit is bad, yet Inbrush Nouveau was considered a top tier weapon when updates for Splatoon 2 ended.

There was a time in Splatoon 2 when Tenta missiles were in the running for worst special in the game. They slowly shifted into being meta as the game was updated. Can you imagine if the devs designed kits around Tenta Missiles being considered bad? What would the game have looked like when they shifted into becoming meta?

The quality of a kit should be assessed on the play-style and variety it provides. Metas will shift, so rather than designing kits to match a meta, they should be designed such that they are enjoyable to play (both as and against) if the meta shifts into favouring them. Basically, criticizing a kit based on the current strength of its individual components doesn't make sense, they should be analyzed based on the style of play their combination creates.

Explosher likely won't find much use for Splashdown, so charging up special with it may feel disappointing: this is reasonable criticism (the style of play feels bad, regardless of the strength of splash down). Saying 52 Deco is bad because curling and screen is bad is just short sighted. For all we know both the sub and the special may be buffed and it becomes top tier. The real question is whether curling bomb creates an interesting play-style with 52, and variety compared to its other kit (I'd argue it does).