For me, it's the knowledge that seasons and stuff don't matter.

I understand why Bungie got rid of most/all of the legacy content, but it also killed a strong driving force for me and likely many others playing the game. For instance, it got me to go flawless each season and I had every Trials emblem besides the scarab because it was my driving force. In Destiny 2, I got one win for the trophy and maybe two more because I didn't want to just ditch my Trial team and I haven't bothered since. Why spend hours grinding for an emblem I'll never wear that will eventually be left behind?

This holds true for almost any snowflake item in Destiny 2. I could do all this stuff and probably will for the MoT, but there is no driving force to keep playing or collecting.

The reason why this is a little different than the Gjallarhorn is because we all have single grinds, be it that, level 30 or the perfect weapon roll, but all these side things are what gave me the most reployability.

The idea that PoE had different bosses for Grimoire cards, getting all the exotics, making sure I got every Trial or raid emblem, etc. Destiny just doesn't have that thrill anymore and I think it hurts it the most.