What would be the reason(s) to not use it by default then? Compatibility? Performances?
AF requires additional texture lookups when sampling textures in a shader. If this is a bottleneck, then yes... AF can have a performance impact. Since applications can often get GPU memory bottlenecked (GPU memory is shared by geometry, texture and other data), we shouldn't force it to be enabled, especially since it doesn't always lead to an improvement in image quality, e.g. when isotropic filtering already provides the theoretical maximum.
I also expect the PR to provide a way for the user to query the maximum AF value and also set it as they see fit. This is a trade-off that can only be made by the application developer themselves.