If "some" GUI doesn't work then don't use it. There are lots to chose from that run very well on FreeBSD.
Sorry, when I used to word "GUI", I didn't mean the window manager. Instead I meant the whole package, from the rendering and communications layer (X), the initialization/shutdown (login support, sleeping, screen saver), the window manager and desktop environment (KDE, Gnome, ...), the basic graphical applications (clock, document previewer, terminal emulation), basic communication apps (mail, IM), and most importantly the office application suite. You interpret "GUI" to mean just the window manager and environment.
Firefox 58 is very fast and totally solid now so any browser based interaction (and that's a lot of it these days) is taken care of.
And Firefox has bugs. As does Chrome, as does Safari. All browsers have bugs. Even today, we still have to switch between browsers to get all forms of browser-based interaction to work. At my previous employer, Firefox was officially the "supported" browser for all GUI platforms, and preinstalled on all corporate-owned machines. In spite of it, there were company forms (like paperwork you had to do for payroll or the human resources department) that only worked with IE and Safari and not with Firefox. No browser is perfect, and you need a collection of them.
Libre Office works very well, so unless you're a Microsoft shill there is no excuse for not using it - other than you just don't want to, which of course is legitimate. 
See below for more discussion of why LibreOffice and friends are not compatible enough. And matter-of-fact, I have tried to heavily use various free *Office solutions myself (still have OpenOffice installed on my Mac), and I installed several hundred copies of NeoOffice on all machines at our elementary school. Claiming that I am a Microsoft shill is laughable. But in spite of having a large bias against using Microsoft's products, I still do use them when they are the best solution available.
Is that a bad strawman argument, or have you really not kept up in the last decade? I don't reboot my FreeBSD desktop system and there is no difficulty with the quality of my GUI.
Sorry, I have never run FreeBSD on the desktop. The story about having to reboot the desktop machine multiple times a day referred to Windows. I gave the example to explain that forcing any person (whether they are an OS developer or not) to use a desktop environment that isn't good for them will lower productivity.
This one used to actually be a big issue for me and unfortunately LibreOffice (or OpenOffice at the time) did not quite solve it. It isn't the fact that I wanted to run Microsoft Office, it is more the fact that everyone else in the office had an obsessive need to use it. I could not guarantee that my LibreOffice documents would display with the correct format when opened up by someone else in Microsoft Office.
Exactly. The sad fact is that a vast majority of all office documents in the world today come from MS Office, and to be productive, you have to be 100% compatible with them, and be able to read, modify and write them. That forces any person who interacts heavily with the outside world to use MS Office. The problem isn't terribly big for Word, because other than minor changes in formatting, the various OpenOffice/NeoOffice/Libreoffice variants are pretty compatible. But where it falls apart is spreadsheets (where many complex Excel spreadsheets won't work in anything else), presentations (where formatting is screwed up when leaving the PowerPoint universe), and database integration (you can't just take an Access database and move it to OpenOffice, it just doesn't work). And even within MS Office, the versions are not compatible enough. Example: For a while, I worked for as startup, and as we were all tired of using Windows laptops, we decided to standardize on Macs instead. So we bought a handful of them. Then we discovered that presentations didn't quite work perfectly when developed on PowerPoint for Mac but displayed on PowerPoint for Windows: the formatting was changed enough (due to font rendering differences) that sometimes words or sentences vanished. Given that our CEO and CTO still had Windows laptops, and given that we were using these PowerPoint presentations to raise dozen of M$ from venture capitalists, we quickly gave up on Mac, and switched back over to Windows laptops. By the way, this was a startup whose goal was to sell Linux-based systems!
I think if Linux ever does become a big player in mainstream computing,
Linux is the dominant operating system in internet servers and on the cloud. In the "top500" list of supercomputers, it has 100% market share: Of the 500 largest computers that are publicly known, every single one runs Linux. I would think that Linux is the 400 lbs gorilla of mainstream computing - except on the desktop (the situation in Mobile is confusing, and the answer depends on whether you count Android as a version of Linux or not, which can be argued either way).
Linux main goal is to replace Windows on the desktop.
When you say "Linux main goal...", who do you mean? Linus? RedHat? Suse? I think "Linux" as a whole does not have a main goal, as it doesn't have a central decision making authority.
See above. X is one part of it, the environment/window manager is another part of it, and then there are plenty of other parts. Fluxbox is just a little part of the overall solution.
... many of FreeBSD developers don't use it on their desktops
and work for Apple™. So they use Macos and do not developing desktop features on FreeBSD,
Two comments. To begin with, I do not believe that the bulk of FreeBSD developers work for Apple (meaning: get a paycheck from Apple), and even less that they work on FreeBSD as part of their Apple jobs and under Apple's direction. I think that statement is flat out wrong.
Second, the bulk of FreeBSD developers are not desktop / GUI developers. For example, take someone who works on file systems in the kernel, or on ethernet drivers. They do not modify the desktop, and probably don't have the wish to do so. They may not even have the skills for that job. What you are espousing here is that you want all FreeBSD developers to work on desktop/GUI features. Sorry, but that's not going to happen.
But I have never even heard of an Apple server before, let alone seen one.
Apple used to sell rackmount servers, which had PowerPC chips in them, and ran the MacOS operating system. They were actually quite price-competitive with Intel-based servers.
Inside IBM, many AIX, DB2 and RS-6000 development departments used these rackmount Apple servers as development machines, because they were much cheaper than IBM's own Power hardware. They were also used for a lot of the early development of Linux on PowerPC, because early on the IBM hardware had a hard time booting anything other than AIX and the "i" operating system (no, not iOS from the cell phone, but IBM's branding of the former AS-400 operating system).
Apple's fibre channel based RAID storage boxes were already mentioned; at some point, you could build a pretty good server cluster using all Apple hardware.
