And, again, was designed in a way that didn't require or desire voice chat.
AND IT WORKED.
Go back two years and tell anyone a successful shooter in 2016 wouldn't have voice chat and could be successfully played internationally.
You'd be laughed out the room.
Like a lot of the pro-Splatoon rhetoric I see on this forum, this kind of reads like it's been written by someone who isn't familiar with the shooter genre as it has existed for the last decade or so. Voice chat is not a necessary component of any modern day shooter, something which is proven over and over by the fact that nobody talks in modern day shooters. Since party chat became common, it's very rare to hear anyone on the microphone, and yet, Call of Duty, Battlefield, Halo, Overwatch, etc. are all popular and work well.
It is also total nonsense to suggest that voice chat isn't desirable in Splatoon. A team that is talking to each other would have an enormous advantage over one that wasn't. While I can perhaps accept that the developers didn't want to implement voice chat inside Splatoon, it is only Nintendo's incompetence in not implementing a party chat system that means you can't easily talk to your team in the game. Also, while Splatoon's design does work well without voice chat, it's no more true of Splatoon than it is of, say, Call of Duty. It has more to do with the relative simplicity of the task at hand and the lack of need to depend on others than anything. In Overwatch you are dependent on your teammates because of the different roles, something only loosely noticeable in a game like Splatoon.
The lack of voice chat in Splatoon is much the same as the lack of good matchmaking and the shoddiness of the netcode. It's more to do with a lack of experience/time/competence on the developer and Nintendo's part than it is some kind of brilliant design decision. For what it's worth, I really like Splatoon, and played it a lot after it launched. It's fresh, but some people seem to think it's revolutionary, which it simply isn't. Overwatch is the best of these three for me -- but I only played Rainbow during its awful beta.