4 airlines fly SFO-TPE right now with up to 8 flights a day (up to 6 of them being around midnight or 1am westbound, so it's not like there's scheduling variety either), and that hasn't had a noticeable effect on fares as far as I can tell. I don't really know what other benefit for consumers there would be besides sheer capacity (although that hasn't exactly translated into a plethora of award seats either) since the Taiwan-based airlines are all pretty similar experience- and service-wise especially if you're in Y or PE.