What really matters is that there isn't a single simple "Cartesian" future tense with only {simple, perfect, continuous} forms.

Here's an example where "will" cannot be substituted with "going to".

"If you sit down then the movie will start." <-- makes sense

"If you sit down then the movie is going to start." <-- ???

In the earlier list, that's "will" because it's a prediction or statement of fact.

True, but also seems like a pretty minor mistake? I feel that I've heard worse offenses from native English speakers. Maybe I feel exceptionally tolerant having lived a year in eastern Kentucky.

Also, "The movie will start when you sit down" is probably better form all 'round.

I'm not clear about the goal of your comments.

https://en.wikipedia.org/wiki/Future_tense#English and https://en.wikipedia.org/wiki/Going-to_future are clear that there are differences between the uses of the different ways to express the future tense in English.

Quoting the latter link, "in some contexts the different constructions are interchangeable, while in others they carry somewhat different implications."

Do you disagree with that assessment? If so, why?

"Don't get close to the bomb, it will explode." implies the bomb will explode if someone gets close to it.

"Don't get close to the bomb, it's going to explode." implies it's going to explode (no matter what) and that people shouldn't be nearby.