Unfortunately some of them, like captions for images / charts / diagrams, numbered headings and the like were really useful
similarly, slack can be pretty useful, but I doubt it feels revolutionary to anyone who grew up using IRC.
It’s more than just this. There is a big difference between authoring parts of the document alone, and being able to see the current state of the document as a whole at all times live while people are writing it.
Do you mean to say that collaborative document editing is perceived to be more efficient but that it is in fact not more efficient?
Assuming that is what you meant, that may be true, but for me when working on a document with others there is more to it aside from saving time on writing together. Specifically, being able to see the document as a whole means that I can write sections of text that are more coherent with the rest of the document, and also that I can suggest changes in other people’s sections while we are working on it.
Collaborative editing is also useful when discussing a document on teleconferencing or when we are in the same room. For example if we are talking about a budget and we all have the Google sheet open I can point to a cell or group of cells, say something about it and make a change immediately in accordance with what we conclude, and they can make concurrent edits elsewhere in the sheet for other parts of the sheet that are affected by our discussion.
Concurrent commenting, discussing, suggesting and accepting suggestions / amendments is mightily useful.
It's honestly tough to remember just how bad check in check out document control was a few years ago. Remember the days when one person did their section in a totally different style from the rest of the document and you only discovered it at 3 am the morning it was due because they waited until the last minute to email it to you and then at 5 am when you had finished reformatting it you realize that all his references to another person's section are incorrect because he wasn't at the meeting where that person said they reorganized their part, so you fix that until 7 am and then you go to add in the bibliography but the guy who had that last sent an old version in his email so you have to call him and get him to send you the real version, and the next thing you know it's 9 am and you need to give a presentation on this 400 page document after getting no sleep and not even having time to grab a cup of coffee.
I honestly don't understand why anyone wouldn't take advantage of concurrent editing.
Such systems make any sort of global changes extremely taxing (so many people need to sign off on it, and there's going to be loads of bike shedding), and naturally favors heavy compartmentalization. The work winds up divvied up among many people working in parallel, but they're not collaborating - you're waiting on mike to finish his portion, not working with mike to make a better document.
It's also very difficult to efficiently divide labor - for example let's say you have an equation heavy document, you could have one person do the first half and another do the second, but it would make more sense for one person to do the text and another person to do the equations as that way neither of you have broken flow. This is possible but a real pain in the butt to do simultaneously with check-in check-out, alternatively you can have the one person do the text and then hand it off, but that might be very suboptimal if you are time constrained. This sort of work is a breeze in a modern collaborative environment though.
Finally, from a true document control perspective, the old check-out method seems sensible as you know what changes were made between when it was checked out and checked in, and you know who checked it out, however you don't really have any better knowledge than that. With a modern system you can see who typed what and you can see every edit they've made, including when they've typed out a long section then decided to delete it and try something else. This can be incredibly useful for keeping track of how an idea evolved. It's also easy to see who actually contributed what: while traditional document control will tell you that so and so made a change to some section, if the change was just pasted in it's difficult if not impossible to tell what the difference from the previous version was. Finally while traditional document control focuses on limiting the chances of a mistake, modern methods allow you to fix mistakes easily.
Now I'm not saying that there's no way to get what you need with check-in check-out document control, but we've reached the point where you don't need to make your work compatible with the process.