The reasoning behind the present progressive representing a future event

I disagree. I see it as the same of the use of the simple present for the future ("I leave next week" is also valid). "I'm leaving next week" in no way implies that "I'm leaving today" in some sense; in fact, it has the opposite implication.

To me, what it seems is that the use of phrase specifying the time, such as "next week," establishes the reference frame of the sentence, and the present interpretation takes place relative to that reference frame. It's like "Imagine that it is next week. At that time, the following statement will be true: "I'm leaving.'" Kind of like the use of the "narrative present" with a time label attached, like in a script or something

Next week: I'm leaving.

This explanation also has its problems. We don't usually say things "I'm leaving last week", even though it's possible to imagine narrating an event in the past. I think this may be because narrative past exists and is naturally used often with past-tense markers.

I did find an example of present + "last week", though, which is maybe similar:

Funny how I say I'm leaving last week to come back and find this place gone to hell and not just my name being used but your and Gregg making endless false accusations that I'm guilty for it all.

ulTRAX reply to Modavations • 5 years ago - comment on "Small Biz Owners On Job Creation", http://onpoint.legacy.wbur.org/2011/12/07/small-biz-owners

How I would explain this is, narrative present can be used to describe past events as well, but it requires more context (a longer passage of narration) since by default people assume narrative in the present tense is non-past, since it could be but isn't marked for past tense.

Probably, there are a number of people who would find problems with my explanation, and there might be evidence against it that I don't know of. The fact is, native speakers don't really know this kind of thing automatically. People just use the present tense in these contexts because it sounds natural.

CGEL and OMEG argue, as I understand it, that the, uh, “progressive futurate” is not aspectual, by which is meant this verbal expression is not referring to an ongoing action. I agree with this.

There are many ways in which I can express the idea that next Friday I will be going to Europe, including "I am leaving for Europe on Friday."

If a friend wants to call me on the phone on Friday, I say I can't do that because I'm leaving for Europe on Friday.

I can't go to the ball game on Thursday because I'm baking a pie for the bake sale and I'm going to the doctor on Thursday. To say "I will bake a pie on Thursday" doesn't convey the same meaning at all.

A friend wants to get some of those nice textiles from Italy, and I say I'm going to Italy in August.

What all of these examples have in common is that at the time when I am being requested to do something else, I will be in a state or condition of leaving for Europe, baking a pie, going to the doctor, which at that time will be an ongoing (progressive) action, or, more simply an action underway.

There's a subtle difference beween saying: I leave for Europe Friday, I will leave for Europe Friday, I am leaving for Europe Friday.

It seems to me there's a desperate effort to make things fit into prior patterns with all this stuff about people not really being in the process of crossing a street or baking a pumpkin pie, which in fact they are. But "I am going to Europe on Friday" expresses something that will happen in the future that will be at that time on ongoing activity. I agree that the activity has not yet begun--it's not the "plan" that's important, it's what the verb says about what I will be doing in the future.

When we use the present continuous to talk about a plan in the future, it is called a Future Arrangement Present Continuous in English grammar. It is used to emphasis that you are certain that something will happen, to the point that you can talk about it as if it's already happening in front of your eyes.

BBC Learning English explanation

Both "will be leaving" and "is leaving" are correct, but the latter implies more certainty, and is less formal. The Present Continuous for Future Arrangements is usually used in spoken English.