Pat expressed the problem perfectly. It is very difficult to know in advance exactly which mountain lifts and what daytrips one might want to do; so much is weather -dependent.
Twenty years ago when we went to Switzerland for 2 weeks of hiking, based in 3 villages, it was easy—-a Flex Pass for 4 days covered our travel between the airport, all the village bases, and back to the airport. And at that time, for a few years when we were visiting Switzerland multiple times, a Flex Pass served as a Half Fare Card in between the validated days for full use. So it was easy.
Now there are lots of options, between consecutive day pass, flex pass, and the various regional passes. I am pretty much worn out with trying to help people with individual itineraries; my general advice is to get a Berner Oberland Pass, with a Half Fare Card, if you are spending 4-5 days or more in that region and want to ride lots of lifts. Or it you are spending 2 weeks in Switzerland and traveling around a lot, the 15-day Swiss Travel Pass might be beneficial for trains and boats, but it won’t fully cover all the mountain lifts.