Who searches for mileage runs on random airlines? The vast majority of people out there are primarily doing it for status (particularly now that RDM's are spend based on most airlines) which means you are going to be searching for flights on a particular airline or alliance and Google flights already has that capability. My bookmark for Google Flights has the Skyteam filter preset as those are the only flights I care about.
I credit to SQ, which partners with quite a few non-*A partners (e.g. B6, AS, VA/VS/VX) and earns nothing on most discount *A fares (e.g. LH group, NH, NZ, all earn nothing) so I end up having to manually filter quite often.