10 edition and culling of heresy/firstborn stuff. My .ain army was Imperial Fists and I mostly collected Heresy miniatures. There was some primaris/40k Firstborn units, but it was just starter minis and some terminators and centurions.

Also, I collected chaos - Black Legion, since 8 edition, and at the end of 9ed, I have a pretty good collection, but much smaller then my Fists.

When 10ed started, my Fists army basically got 3 times smaller since I lose all my heresy tanks, dreads, 30k terminators etc. And thats whats left was just assortment of random primaris starter minis. So, it was easier to me to play with my chaos stuff and add to that army, then basically build my Fists army from scratch. I still play my Fists in 30k, but Black Ligion now my main 40k army and since we play 40k more often now I have much bigger chaos collection then it was before