[Wowfunhappy]

As much as this bothers me because of who I am, I don't think it's a real problem. Those files shouldn't take up significant space unless the developer is doing something stupid.

It might be nice if macOS had some sort of automatic cleanup routine when an app is trashed, but that would either require showing the user an extra dialog (a la AppCleaner's) or introducing an opaque system which could potentially lead to data loss.

[microtonal]

Indeed, data outside the application folder usually consists of a preferences plist and saved application state. Of course, there could be caches as well, which could take up a fair amount of disk space.

But I think the primary argumentation in favor of what macOS does now on drag-to-trash is that the users preferences are preserved, for when they install an application again.