But what is the point of using Haiku if you are going to run (desktop) Java applications? The main appeal of it is its native API, use of multithreading everywhere and how integrated the applications are. If you are going to treat it as just a window manager with a different theme, then why not just use XFCE - it even has a BeOS-like theme.
Which happens to be the fallacy of GNU/Linux desktop as well.
The beauty of each desktop centric OS is the whole platform, the set of SDK languages, the respective frameworks for exposing the OS and hardware features that make the desktop experience unique, and IDE developer experience.
If it is to run desktop agnostic apps, then anything can do.
There's always been some leeway in that. Both the original Mac OS and Windows had a variety of languages targeting it, but they tended to strive towards emulating the core SDK experience (if they weren't a wrapper around it).
Unix had it different with there being no core experience in the first place -- X11 being a glorified terminal multiplexer plus xeyes. Whether it was the commercial Unices with OpenLook/NeWS vs Motif/CDE or later Linux with Qt vs Gtk. Not that there wasn't a chance -- at one time Motif won over the commercial sector and if it would've been available with the same license as Xlib/Xt, GTK would've never been developed.
But that seems to be a thing of the past. These days the web forced us to give up all platform-specific UI expectations -- heck, any expectations of having a proper UI in the first place. And thus applications are all over the place, Electron being just one particularly heinous culprit. Material UI on the desktop. Plenty of applications starting up web-based interfaces in the browser.
On the one hand, as you say, this doesn't make the particular desktop matter anymore, it's more a vehicle of getting your wifi driver to work properly and load the browser. So one could use Linux or even Haiku without any big losses. On the other hand, if everything's pastel-colored oatmeal, there's no big benefit from a neat, unified, C++ desktop experience that BeOS was aiming for.
If you want a vision of the future, imagine HTML stomping on your face -- forever.
Hence the opinion Steve Jobs had about UNIX, NeXTSTEP's POSIX support being more geared towards bringing software in and win DoD contracts than anything else.
I don't need such vision, it is how it would look like if all OSes happen to jungle browser instances, ChromeOS style, while a big chunk of applications runs on someone else's computer, abstracted via language runtimes.
The return to mainframe's timesharing days and very sad outcome for desktop computers, with the minimum experience common to all vendors.
Graphical APIs not able to take advantage of hardware, hardware support that takes years to have minimal APIs, drawing UIs pilling divs customized with CSS and JS to mimic visual behaviours.
It's not a pretty vision of the future but maybe there are ways to escape it.
The reason apps all went cross platform is that desktop operating systems stopped innovating. Which is the desktop platform with the most new native apps, where developers take pride in making non-cross platform experiences? MacOS for sure. Which desktop OS has been consistently the most innovative over the past 20 years? MacOS!
Haiku running IntelliJ is technically interesting, but ultimately as a user I don't understand why I'd want to use Haiku. BeOS may have been innovative 20 years ago but it's a dead OS that hasn't changed in decades. May as well run IntelliJ on any other platform, then.
I think the desktop can be great again, but it will take a new generation of developers with new ideas about where to take operating system technology. I also tend to think that whoever tackles that problem will probably tackle it by writing a cross-platform app runtime that slowly expands to become more and more of an operating system until the actual OS is indeed, just relegated to providing some drivers and bootstrap code. But the actual UX will be defined entirely by that environment. Sort of like how Pharo gives you a full blown desktop environment when you start it (but I'm not arguing for Smalltalk)..
