Tier 2 support is still support, and there are a lot of tier 2 targets. Seriously, look at the list:
https://doc.rust-lang.org/nightly/rustc/platform-support.htm...
There's nothing wrong with saying, "Look, I'll do everything I can to help you, but you're using a PowerPC on a $5000 FPGA board that I don't have, so the things I'll be able to do for you may be somewhat limited." It's super common to see kernel developers responding to bug reports stating that they need more traces from people with the affected hardware configuration. How is this any different?
When it comes to workstations or web servers, what the GP said is true: everything outside of Windows/Mac/Linux x86/arch64 (and maybe BSD) is extremely niche. When it comes to the space of all systems where C runs, that probably isn't the case. When it comes to the space of all systems that Rust (and packages that depend on Rust) should support (for some definition of "should")? Who knows.
There are two questions:
a) What kinds of systems should be targeted (by Rust itself and also downstream packages/distros)? Hardware from the past ten years? Twenty years? Forty years? Embedded devices? Industrial control systems? Mainframes? Obsolete workstations from companies that have been dead for decades and are only used because one other piece of crucial software requires them?
b) What percentage of the devices in those spheres are actually unsupported right now? If you told me that purely in terms of number of devices, Windows/Mac/Linux x86/arch64 covers 90% of the devices in the world, I would believe you. I would also believe you if you told me those only covered 40% of the devices in the world. I genuinely don't know. And this question has to be asked in the context of the answers to (a). Our subjective impressions of "how wide of an impact this has" are at the heart of the argument; we don't have a common truth. I think a lot of the emotions being seen are by people for whom it feels like their entire world is excluded. That alone isn't a basis for project scoping, though it's worth listening to.
Traditionally C's mantra has been that it runs on anything with an instruction-set (maybe inconsistently, but there will be something available!). That's cool! But if that's the bar for a new systems language, we will never ever get a new systems language because C has half a century's head-start. We can debate whether a specific package or distro was premature to require Rust, which is highly dependent on their target market of users (and not really the responsibility of the Rust community itself!), but no matter what, Rust itself is going to have a cutoff, and that alone isn't a valid reason to dismiss it.
Once you get down from the obvious large players people tend to consider what they do mainstream enough and what they don't do too niche.
Say the same again about embedded devices and the Netflix network backend if you dare. Also, some compiling toolchains for consoles based on BSD os'es.
Embedded devices are MUCH more than the 90% of the world. Not eveything is a cumputer.
Add routers, switches, and mobile devices and now your "90%" goes out of the window.
This sentiment of "Just use what is most popular!" is pretty common, and it always makes me sad especially on a site like this. Not supporting operating systems outside of the absolute mainstream also sends the signal of a lack of interest in developing new things.
I'd argue that valuing portability and making it as easy as possible in general comes with more benefits than just "supporting those unpopular platforms I don't care about", typically the users of those platform will come forward and do work if only you welcome them. Not focusing on niche systems in particular I can understand, but what speaks against making the porting easier for the willing?