/usr/local is supposed to be kind of a "standard Unix tree" (with like bin & share directories and the like) that contains software manually installed by the sy

/usr/local is supposed to be kind of a "standard Unix tree" (with like bin & share directories and the like) that contains software manually installed by the system administrator, for the use of all users, possibly from multiple sources / by multiple techniques. If one source of software or user takes over the whole thing, they are muscling out all the other valid use cases for that tree. /opt is supposed to be a place where programs that don't want to co-operate are granted ownership of one directory to install stuff in, which they can organize as they wish. It's often used as a way to install badly-packaged proprietary software or to give "foreign" package managers a place to put their packages. Brew wanting to take over /usr/local is kind of like a package manager wanting to take over the whole /opt. It's overstepping its bounds.