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 suspect there are a number of people on FT who love to travel to new destinations and don't want to deal with ULCCs and have the slim opportunity of acquiring low level status on one of the alliances. Then there are those who aim for status on multiple alliances since they live a markets where there's no clear winner on which alliance to choose (i.e. ORD with AA and UA or NY). Finally, then there are airlines that people travel on that aren't affiliate with any alliance like Alaskan or Emirates where accumulating status/RDM can be quite lucrative.
Safe Travels,
James