The GNOME devs are actively closing the software as much as possible; their software philosophy is probably as closed as you can get while being formally open source.

There are many interesting informations around to get a picture of how they think; this article has interesting concepts:

https://igurublog.wordpress.com/2012/11/05/gnome-et-al-rotti...

and this is the most relevant part:

  I think one of the most important cases against applets (as they are currently defined
  in GNOME) is that they are extremely detrimental to the Identity of the product or 
  platform. [...]
 
  Let’s say that we are trying to define either a product or a product platform. I don’t
  think it is possible to do this without some “brand” coherence. And it is arguably
  impossible to do this effectively with such an ad-hoc/individually-customized design
  identity.

Wow, WTF. I could sort of see that type of thinking from a commercial product marketing manager, but it's puzzling coming from an open-source developer. All that flufftalk of "brand" or "identity" --- whatever happened to just designing products that your users want to use!?

It is particularly perplexing since Windows before 8.x was far more customisable and themable, which probably lead to its success.

Windows was not successful due to its customisability or themability. Windows was successful because in the early days it was the best (cheap + capable + commodity hardware) choice, it built an ecosystem of apps, and then it ruthlessly preserved as much backward compatibility as possible to carry the advantage forward.

The lesson of backward compatibility is slowly getting lost across the whole industry.

When GNOME 3 came out, it felt like they had this vision of a Better way to Do Things and didn't see why all their users wouldn't be excited to change their workflow. How they responded to criticism definitely makes sense given those thoughts about Identity.

And Apple's un-customizability.

I gave a try to KDE at the beginning of 2014. I used it for a few months but everything was painful: twice as many clicks to get things done than I used to in Gnome. It reminded me of Windows.

I installed Gnome Flashback and stayed there. I'm customizing Gnome Shell to look like the old Gnome 2 now. I have to sort out a couple of problems with the Nvidia driver (flickering unless I use a strange 40 Hz refresh rate) and I'll be there for the next few years on Ubuntu 18.04 LTS. If there won't be themes anymore by then, I'll switch to something else because the Gnome developers and I have two really different ideas of the desktop. Ironically I'm closer to KDE but it was really unusable.

You really should give KDE another go. KDE5/Plasma is the most coherent UI available on Linux, by a long way.

KDE Neon is a KDE-focused distribution, using Ubuntu LTS as its base, with a pleasingly minimal default install. Every Ubuntu PPA I've tried works just like on base Ubuntu. Similar to Linux Mint and others, it works as a live distro so you can check it out before you commit.

KDE4 made me switch away too (I chose Linux Mint), it simply felt too much like a transitional release cycle. KDE5 brought me back, big time.

And when Brise actually makes Gnome extremely beautiful for me. I find adwaita dull, as well as the tango icon set.

Anyway, for consistency, I hope using the Brise theme and the corresponding icon set on gtk apps in KDE remains possible. Adwaita would probably not work really well on KDE apps.

More generally, theming gtk apps is needed for nice integration in many non gnome environment.

> one of the biggest advantages of running desktop Linux was its easier and wider customisation.

The fact that GNOME is less configurable does not mean that the Linux desktop is. Gnome is not compulsory, you can easily run whatever window manager or desktop environment you fancy, and still run--or not--Gnome programs.