Hi!
I have been using QEMU since 2019 to emulate all sorts of operating systems I have ever recognized in this life, and I can say some things...
OS 9 under QEMU Screamer fork is pretty much good. Okay, I am limited to 512MB RAM (or else the sound would be crackling/stuttering); No 3D acceleration cards yet (despite of an early attempt on e-maculation forums from other users to try and bring PCem/DOSBOX's Voodoo 2 emulation, and ATI Rage 128 emulation). But runs nearly everything I throw at it. And I am in love with the OS 9 UI/UX experience.
Regarding the low selection of graphics card on QEMU, one can always do a GPU passthrough with a real GPU card. But I'd rather see more developers working on QEMU code to bring other graphics cards. For example PCem and 86Box has an impressive amount of graphics cards and Voodoo cards to emulate, and that could be ported to QEMU with the right people, IMHO.
But apart of that, OS 9 performance is pretty good for whatever I want to do with it. QEMU also performs well on M1 Macs (I have a M1 MacBook Air 2020) to run ARM versions of Ubuntu and Windows, as well as emulate classic i386 versions of Windows (3.11, 9x) and I am even emulating x64 Windows 10, because I couldn't leave it without a few select work related stuff.
With a fork of QEMU that has MESA/Glide passthrough, I can play some 3Dfx Windows 9x/XP games from late 90's & early 2000's. Not 100% perfect, but very good and VERY playable (Need For Speed II SE for example, can run at 60fps without a problem).
The only thing I couldn't emulate yet was the intel OS X builds, because they need the hackintosh methods to work there (I have seen some success stories) and although I did have some hackintoshes years ago, I haven't done any since 2016, so I am out of that scene because I prefer the real Macs, so yes, in a way I can understand the OS 9 fans feelings better; my decision to stop with hackintoshes and move back to real Macs was purely based on performance and maintenance reasons (Hackintoshes require more).
I have spent nearly 4 years trying all sorts of emulators for games and other nostalgia related stuff, and I have found that QEMU is the most powerful option to do that. Can it replace the real deal? It depends of what you want to do. There are linux gamers using QEMU for bare metal Windows virtualization to run Windows only games with linux hosts, and GPU passthrough, for example. This is the most extreme example of usage for QEMU. There are QEMU frontends to do that job, like UTM (for iOS and macOS) which is a very, very good and free option (VMware and Parallels are outrageously expensive).
The answer to "can it replace the real thing?" is probably a strong NO, the real stuff is always better. But when you can't afford a real PowerPC Mac, is more interested in the software side of the things than the hardware, or can't take one with you in the middle of a move to Spain in 2022 (like me), QEMU is a savior for me. OFC I still have the idea of getting a real PPC Mac to play with, but I delayed these plans for some good time due to $$$ and other time constraints. I'd love to have a Mini G4, or a G4 Cube, or a Tangerine iBook. As an UI/UX Designer myself, their designs are interesting to me.
We can see QEMU as an option, not a solution. I agree with Jubadub that we should embrace both worlds (virtualization and the real thing). QEMU can be used for OS9 tests before doing the stuff on real PPC Macs, for example, so you can have a rough idea of what will happen there before. I know it does not 100% replicates a proper PPC Mac, but it can give a good idea of what to expect from that.
If anyone is interested, I can share in other thread my experiences and findings with QEMU running PPC OS 9 and PPC OS X. While OS 9 runs fine, PPC OS X still does have some sound issues, but they're being worked on as we speak by the devs of Screamer QEMU fork, whom expect to be able to merge the patch into mainstream QEMU when they improve it. Apart of sound, PPC OS X under QEMU works pretty good. The older the OS X version, the faster. Tiger being the absolute beast of them, even though Mac fans would prefer Leopard (myself included).
So, this is my experience with QEMU. For me it is just a hobby for nostalgia reasons and an all in one solution (I can do it all on my M1 MBA, be portable, and I do not need other computers).
Embracing the emulation/virtualization of OS 9 will not hurt it, or the real PPC Macs, in fact it can just make it stronger and more accessible to more people, maybe bring in some good hobby programmers who can develop some apps like Newsstand for OS 9, por example. Isn't amazing that Newsstand was developed in 2021 for an OS that is more than 20 years old? This can help to keep OS 9 still alive for many of us. Maybe the forum needs a proper forum section dedicated to emulation. The development and findings there can help to improve the real stuff on PPC Macs without hurting them.