I am not advocating for choosing Luzern over the Berner Oberland; I am suggesting it would relieve some travel stress to head to Luzern for the last night, to make it easier to get to the airport in plenty or time for their flight. They can take the train to Luzern after a full day in the Berner Oberland; as long as it is still light out they can relax and enjoy the very scenic journey along the lake (Brienzersee) and over Brunig Pass. There are hotels just outside the Luzern train station so it is not a long walk to the new lodging.
Can they reach the airport from Wengen, etc. in plenty of time for their flight? Yes, they can. But then they will likely be on the faster route via Spiez and Bern, and in summer that train can be very crowded, with no seats left
We experienced that in July 2018 when we were meeting our son’s’ family on the train between Bern and Spiez on our way to Muerren. We boarded in Basel and they were coming from Zurich, so they boarded our train at Bern. I had reserved seats for all 6 of us in Car 5, but the platform was so crowded they could not board that car, they had to get on the very last car and then try to make their way forward a very crowded train with their luggage and 2 6-year-olds. It took them half the way to Spiez to reach us, and in the meantime I had to get pretty fierce about defending their reserved seats, not knowing if they had even made it on the train (I could not spot them on the platform).
I knew to receive the seats on that Intercity train because we had a bad experience with a crowded train on our way to Zurich airport from Brig on a previous trip——I think it was our 2007 Swiss trip but could have been another. There was some big event in Zurich that day (unbeknownst to us) and tons of people were heading to the city for the day. We never did find seats—-we spent the whole ride standing in the open space at the end of the car.
And on yet another trip there was track work that required a substitute bus for part of the journey—-which meant getting off the train, boarding a bus for the transfer, and then getting back on another train to Zurich, changing at the Zurich main station (which can be a challenge). When I originally planned our route to the airport that day there was no construction noted on the website, and I chose a train that went directly into the aiprort from Bern. The best laid plans. . . .
The Swiss rail system is VERY reliable, and it is not that I don’t trust it to be on schedule—-but tourism has become so popular that the trains are much more crowded these days than they were when we first started going there for our hiking trips in the early 2000’s.
Of course with all the covid restrictions and fewer tourists (probably none coming from Asia), it was not crowded at all in 2021, but who knows what 2022 will bring?