Not sure about always..? That came in with the Mac App Store (and iOS before, but no plug-ins there).There was always the lingering danger and implications of sandboxing and the resulting privileges for file access and shared memory.
True, yet never such an issue on other platforms.. there's nothing special about macOS to make it more or less vulnerable to such things happening, but it seems to happen more anyway (ObjC class name collisions, problems unloading dylibs for a long time).. perhaps this overall approach to building applications just isn't as high priority on Apple's radar compared to those working on the equivalent parts of Linux and Windows.There have always been possible issues with conflicting libraries and namespace collisions.
For "big" plug-ins you're probably right - have your big VIs which need non-trivial filesystem access and other privileges run as separate processes; keep smaller and simpler things which don't need any special permissions in the main DAW process for efficiency. AUv3 seems to be headed in that direction?I'm think that maybe the approach of application based extensions is something worth embracing. Haven't dealt with it yet, though.