It’s not really that costly or inconvenient to extend their stay on ISS compared to Skylab though. With Skylab 4 you had to somehow figure out how to support fo

It’s not really that costly or inconvenient to extend their stay on ISS compared to Skylab though. With Skylab 4 you had to somehow figure out how to support food and water for half-again as many astronaut-days as planned but ISS has routine supply ships and a big enough permanent crew that adding two extra people isn’t a strain. From the perspective of ISS operations, having an extra two crew members more than you planned also means you can get more work done. And if you’re one of the astronauts, who has worked hard for the dream of going to space, getting a bunch of extra space days is not exactly an unwelcome surprise either. So not only is it not actually a big deal to keep them on the ISS for longer, but I bet a few people are happy to have an excuse to keep them there.