I don't think I've ever seen a bluetooth speaker that had a physical volume control that was an actual analog volume control, it's always a software based control.
I've had a large power amplifier with no volume control connected to my PC for many years, at least back into the Win98 era. Windows has always been in control of the volume, I've never had any issue with it at all. About 12 years ago I built a USB volume/mute control that sits on the desk to make it more convenient. It works perfectly fine, I suppose it's technically possible but it shouldn't happen and I've never had it happen. Unless you're driving a massive power amplifier and/or have valuable and easily damaged speakers this is a total non-issue. The remote device volume control acting on the master volume on the PC/mobile device is exactly what I want, and exactly how most people now expect it to work.
It's not just Windows you have to worry about. It's the software you use for audio, too. If the Master Volume control works to limit volume, then you do have some protection from software. I've seen software max volume after an update or patch. As for streaming, same thing there. You're not in control of how loud the streaming station is broadcasting, be that fro a .pls, Amazon, Youtube, etc. And, again, to your point, Windows Master Volume can be used to limit absolute volume.
As far as BT devices having a volume control knob not seen in Windows as a "device," they are all over the place. Maybe they are all changing over to a non-independent volume control? Probably. My BT enabled amp has a physical volume knob, and bass and treble too, and the volume it is not identified by Windows as a "device." Old amps with volume knobs that have a line in can use modern BT receivers. Those volume controls are not affected by software.
To your point, having one master volume control for everything is a great idea. I just don't trust Windows software to get it right in mission critical instances. If it were software integrated into hardware, such as a BIOS, then I'd be more likely to trust it.
I'm glad that having Windows software control the main volume on your audio equipment serves your needs. However, even if "most people" want it that way, it doesn't mean it is better. For instance, most people just want to listen to music trough their phones on a single, mono BT speaker. That doesn't mean it's superior to other options. To be clear, I'm all for a functioning master volume, just not one that takes away my physical audio control - yet.
For me, I want a way to physically cap power output to my physical devices. I'll use and do use Windows Master Volume all of the time. But, currently, I still want absolute control over my hardware. Maybe I'm just "old fashioned?"