I'm not BIS and I'm not in a progression group. I don't even play current content very much so there's quite a few encounters I've never seen, let alone cleared. The reason I play is twofold, really.
1. On a personal note, I love collecting things such as glamour, mounts, orchestrion scrolls etc. I love crafting. So I'm happy to potter around doing my own thing on nights when nobody else is around. I don't go crazy with clearing everything on day one so there's usually a decent amount of stuff to do right up until a new patch is approaching.
2. The other reason I play is friends. I'm in a tight knit group of real life friends and we run old content since you can't undersize current content and we don't have eight regular players (I don't have eight RL friends who want to play FFXIV so that's never going to change). I'd love to take it a step further and work on current content too, but since that's impossible we grab strangers to clear the mandatory stuff then go back to old content for now. When I did Cape Westwind back in 2.x at the correct level, synced, in a full party, it was a total faceroll because it's undertuned story content. When a new member joined our group and we decided to do it at minimum level with half the intended number of people, it was a fun fight with wipes and positioning and roles and strategy. Naked blind Coil runs were hilarious back in HW; none of us watched any guides so we had to figure out all of the mechanics from scratch whilst also accommodating new problems caused by our low numbers. If I'm not in early progression the first time around I honestly miss a lot of mechanics and challenges as people gear up so fast that they trivialise fights after a few months; our lowman runs make me appreciate content in a more long-lasting way.
So in other words, I don't tend to run out of content since we take things slow. Despite playing a reasonable amount. We'd have even more content if not for the weird blockade on joining anything remotely current without having the right number of people - since a lot of it would be unclearable we'd be kept busy for months!
I'm a filthy casual scrub, but I have fun. If I stop having fun I'll move on.