I want adjustment for all screens too. But the legacy set exists for a reason, which is to prevent regressions for API users that haven't adjusted to per-display controls yet.
OSD is one such usage, and there is an argument that we shouldn't adjust brightness of more screens if we can't yet represent multi-display brightness changes correctly.
Dimming is the other one I can currently think of. The proper fix will be !360 or something of that sort, but that still has a bit of work ahead of it. Until that happens, I'd hate to regress users for whom the current dimming code worked well enough but might get regressed by the dimming code ignoring the current (secondary) monitor brightness.
I would argue that the fix for this indeed arbitrary split is to fully migrate to per-display APIs, and then remove the concept of legacy displays from the code entirely together with all the corresponding API. But the migration must be careful, to make sure that the new code is indeed flexible enough to handle the extra complexity of per-display APIs. Until we can guarantee that, I'd rather leave things as they were and avoid exposing users to awkward half-way bugs. We need to fix things already, but let's not introduce a different set of bugs while refactoring.