This feels like a failing on Microsoft’s part, at least for current versions of Windows. The bar for a viable system font has been raised and it’s their responsibility as an OS vendor to ensure the bar is met across localizations.
As for old versions of Windows… well, one should probably look at their target market/user base. If your demographic is firmly Roman western use of system-ui is probably fine — as far as I’m aware, the bulk of remaining users of XP/7 are in East Asia, chiefly China.
You can call it a failing on MS's part up until you have real users that are impacted because then it really doesn't matter who's fault it is when users can't use your site.
So MS should fix this and until then you shouldn't use system-ui.
This problem isn't limited to Windows 7/2008 or XP. It affects Win10 too.
>especially when the font is merely ugly in a cosmetic sense, and not unreadable
Eh, you won't go very far with this attitude in any design team.
Also, anyone who tried to manually craft font-family fallback path are already making effort. You can literally choose to not assign any font. Actually, it probably works better: most of browsers have sensible default (which often times is system font) already.
My counter-argument would be that even if a platform is EOL, if it represents a serious chunk of your audience then you should probably do what you can to accommodate them. Especially if, as here, you might be able to fix it with a very simple change.
