I'm not used to seeing informed takes on the games industry on Hacker news, but then I looked up to see your name. Love your stuff, your website was mind opening when I was in college.
Per the separation, I think this was far more common both in older unity games, and also professional settings.
For games shipping on mono on steam, that statistic isn't surprising to me given the amount of indie games on there and Unity's prevalence in that environment. My post in general can be read in a professional setting (ie, career game devs). The IL injection is a totally reasonable consideration, but does (currently) lock you out of platforms where AoT is a requirement. You can also support mods/DLC via addressables, and there has been improvement of modding tools for il2cpp, however you're correct it's not nearly as easy.
Going to completely disagree that Burst and HPC# are unnecessary and messy. This is for a few reasons. The restrictions that HPC# enforce essentially are the same you already have if you want to write performant C# code as you just simply use Unity's allocators for your memory up front and then operate on those. Depending on how you do this, you either can eliminate your per frame allocations, or likely eliminate some of the fragmentation you were referring to. Modern .Net is fast, of course, but it's not burst compiled HPC# fast. There are so many things that the compiler and LLVM can do based on those assumptions. Agreed C# strings are always a pain if you actually need to interpolate things at runtime. We always try to avoid these as much as we can, and intern common ones.
The fragmentation you mention on after large operations is (in my experience) indicative of save/load systems, or possibly level init code that do tons of allocations causing that to froth up. That or tons of reflection stuff, which is also usually nono for runtime perf code. The memory profiler used to have a helpful fragmentation view for that, but Unity removed it unfortunately.
In general when game development comes up here I tend not to engage as professional gamedev is so different than what other people tend to deal with that it's hard to even get on the same page, but seeing as how this one is very directly dealing with my expertise I'll chime in.
There are few things off with the this post that essentially sound as someone more green when it comes to Unity development (no problem, we all start somewhere).
1. The stated approach of separating the simulation and presentation layers isn't all that uncommon, in fact it was the primary way of achieving performance in the past (though, you usually used C++, not C#).
2. Most games don't ship on the mono backend, but instead on il2cpp (it's hard to gauge how feasible that'd be from this post as it lacks details).
3. In modern Unity, if you want to achieve performance, you'd be better off taking the approach of utilizing the burst compiler and HPC#, especially with what appears to be happening in the in sample here as the job system will help tremendously.
4. Profiling the editor is always a fools errand, it's so much slower than even a debug build for obvious reasons.
Long story short, Unity devs are excited for the mentioned update, but it's for accessing modern language features, not particularly for any performance gains. Also, I've seen a lot of mention around GC through this comment section, and professional Unity projects tend to go out of their way to minimize these at runtime, or even sidestep entirely with unmanaged memory and DOTS.