I agree a wall of text will often not be read. But I think often the problem is with the author for having written it in the first place, rather than the reader for not being bothered. It's like that famous Pascal quote, "if I had more time, this letter would be shorter".
One part of that is about redundancy and verbosity: with some effort it's usually possible to say the same thing with a lot less text.
The other part, as you say, is about knowing your audience: an executive didn't read your massive email because they genuinely don't need to. If you can pick out the few bits of information that they actually need to know then that's better for everyone.
At this point, I explicitly call my "second draft" a "word removal pass". That's most of what happens. If I were ever to write fiction, I'd also write myself a text editor customized to some of my own bad habits, like, it would syntax-highlight all adverbs. I use them a lot in first draft writing but most of them should be removed.
As a sibling to your comment says, I have often unashamedly included an "executive summary". They seem to appreciate it. I also rather frequently pull them off the email chain when I'm diving into technical details, unless they explicitly ask to stay up-to-date on the entire chain. Then, if I want to re-update them, I'll add them back in, so they get the whole chain in one shot if they really want to dig and I'm not "hiding" anything, but they still get their summary.
Upshot, don't be scared to put "executive summaries" in. It's not sarcasm and it's not a joke.
One part of that is about redundancy and verbosity: with some effort it's usually possible to say the same thing with a lot less text.
The other part, as you say, is about knowing your audience: an executive didn't read your massive email because they genuinely don't need to. If you can pick out the few bits of information that they actually need to know then that's better for everyone.