Only the extension pack is not free to use. The base product is under the GPL v2 and is free to use.
You can use the extension pack for personal use but any use in a business context requires a license.
You can use the extension pack for personal use but any use in a business context requires a license.
Also, one could make a fork which just rebrands it and removes all references to non-open source components - a bit like what Oracle themselves do to RHEL.
Even stuff like RDP, I haven’t looked at the VirtualBox code in detail, but I assume there would be some interfaces to isolate display clients from the low-level technical details of the graphics card emulation-so it might not actually require as much “low-level chops” as you might think.
But your last point about not many people want to spend their weekends on it is spot-on: I myself am not volunteering to do any of this. Between my day job and a young family, don’t have as much time for personal projects as I used to, and what time I do have I’d rather expend towards other goals.