There's a reason why everyone uses microsoft exchange, despite all its myriad of flaws, and the flaws of its major client Outlook.
And it's because it offers so much functionality, precisely because it so much more complicated.
It's like saying you can secure your house if you build a 20ft wall round it with no gate.
Sure you can, but it becomes pretty useless.
Like the majority of awful “enterprise” products on the market, the primary reason that it’s popular is because it’s from a megacorp who speaks the language of the buyers, who are all aspiring megacorps. I was horrified the first time I used exchange and couldn’t wait to change providers the moment I had the chance.
So it’s more like saying you can secure your house if you use a security service who sets security targets instead of sales targets.
Sometimes being the least worst option is all it takes.
I call maximum shenanigans on this. Exchange is a fully-integrated groupware suite with a single-pane-of-glass on both the management and the user side. I am aware of precisely zero feature-complete alternatives, let alone anything "better".
So I’m sure that for some huge enterprises, the complete feature set from Exchange is actually necessary, or at least desirable. But for everyone else - including many companies I’ve worked in at a senior level, and almost certainly many of the victims of this vulnerability - I’d call shenanigans right back at you.
And you are right, loose coupling does rule out a very small set of functionality. For example an email sent to a user might have an smb: link, and then Outlook used to do a preview of the email, automatically loading all the links, which would cause your credentials to be sent to the smb:// server just by previewing the email, thereby allowing malicious attacker to steal password hashes by sending emails to victims (no click was needed).
So that would be an example of excessively tight integration and a design philosophy that was fast and loose with shipping both credentials and executables across the network. I think we have learned from those lessons.
In terms of why it is dominant today, it is because of fairly rational C level decisions, not users clamoring for it as opposed to some generic email/calendaring solution. Microsoft still knows how to do support, there is a large pool of cheap IT admins certified to work on it, and it allows you to run your own server instead of buying a service from gsuite. Really if Google could shed their disdain for human beings and learn to think of them as customers, they could take a lot of market share away from Exchange, because right now it is a trade off of security versus support - the functionality is basically the same.
Gsuite email doesn't even have good support for things like delegated access to shared mailboxes, treating them more like a distribution group. On Outlook they appear by magic on your sidebar.
Source: I am currently migrating some acquired users from gsuite to 365
You conflate functionality and complexity. If you think about it for a minute, complexity actually hinders functionality. There is some intrinsic minimal complexity to useful features of a software system for it to be functional. Exchange could be way more useful, if it wasn't so complicated and it could be a lot easier to keep somewhat secure.
Exchange in many circumstances feels more like a banks vault but instead of steel door with a wooden one with the cheapest padlock you can buy and a sign "we go here once a year to check everything is in order" where real banks usually work a bit differently... There are many cases, where an attacker gained access to the complete Active Directory through Exchange. At least so I was told by a company that did the consulting afterwards to clean up the mess.
Of course a server process which is designed to modify (among other things) group memberships needs different permissions than a user, why would that not be the case...?
If you don't like it being highly privileged, don't grant it the permissions. Or hire someone who can.