What about doing a kind of content because you like it in itself and to help other people who needs it to finish it, and not for the carrot you get at the end ? For example i still do hunts and the two special ShB big fates, even though it has been whole months i dont need anymore all the things they give. Thats the closest thing you can get that ressemble world bosses, and i just like to hit them with numerous other players (plus it is always so entertaining to see so much people still dying at Mudman's attacks XD). Do you truly really, as you said, enjoy end game fights if all it takes to demotivate you doing them is the gear becoming obsolete too fast ?

But eh, dont get me wrong, i know very well a big part of the community is rewards driven. Just look at the number of people doing ShB fates now, despite fates being an all time despised content. I did never see so much players doing the previous expansions ones.