To be honest, I'm pleasantly surprised ARMS came out even as unscathed as it did, warts and all.
Nintendo has a tendency for their new IPs to be throwing stuff at a wall to see what sticks and ARMS is certainly a concept that falls into that.
Considering Splatoon initially got criticized for a lack of content and had some lower scores than it could have as a result, I can see ARMS improving a fair bit over time. There's some things that can't be fixed that way, of course, but let's be perfectly honest with ourselves that Splatoon was a game that came into it's own rather than starting perfect and that they're clearly channeling it for this.
While I'm at it, if anyone is expecting Splatoon 2 to have a massive amount of content on launch, settle your expectations down a bit. Having a co-op mode on top of competitive multiplayer and the singleplayer mode is a big content bump due to the juice of Splatoon being in the multiplayer, but singleplayer is deliberately designed to be easy for beginners and so might get a bit weak if you know what you're doing. That and the first game had an excuse plot so bad that games from the 80's are laughing at it, which Splatoon 2 looks to be doing. . .a bit better than so far? The real meat of the plot is in the Sunken Scrolls you pick up. If you hated BotW due to the game allowing you to discover places in any order meaning the plot was a bit thin, then if it's anything like the original Splatoon then Splatoon 2 will drive you nuts. The real fun in Splatoon is that multiplayer is so good that you can play with total randos and it's still as addictive as potato chips/crisps/etc. . .assuming you click with it, of course.