> 1. marketing and reach

In theory, yes, but they have less expertise than you might imagine. For technical writing, keep in mind that editors at publishing companies aren't actually tech people. They may have been at one point, years ago, but they don't really know what matters to programmers today in that way that an active working engineer does.

> 2. financial risk on paper copies

That was much more of a thing before print-on-demand. You don't have to take the risk of a several thousand copy offset press run anymore.

There is maybe an argument that offset printing is higher quality, but I have textbooks from major academic publishers whose print quality is clearly worse than the POD stuff I get from Amazon for my book.

> 3. Production services (e.g. editing and artwork)

This is absolutely critical, agreed. Though they are often contracting out for this and if you're comfortable finding and vetting freelancers yourself, then they don't add a ton of value.