Yeah, it's funny. I bough the original Crusader Kings when CK2 was already in the works. Played it for years. When I finally got bored with it, moved on to other things instead of CK2.
After many years, I finally came back and bought CK2, only to find CK3 already in the works. Deja vu!
I've still got lots of things to do in CK2 that I haven't tried. Lots of playing time left! When I get bored with it, I'll probably turn to other things rather than CK3.
I'll probably come back and buy CK3 just in time for them to release CK4!
I bought the original CK pretty much on release, but I didn't get CKII until 2016. I had a lot of fun with the original even though on balance, it was really pretty much a broken game--there were large areas of gameplay that never worked as intended, but, typically of Paradox products, it did get a lot better when patched. Had there been a couple of more patches/expansions, I would have probably been more willing to buy CKII sooner; I felt like Paradox had abandoned CK before getting from being a good but flawed game to a state where it was a great game. That, and a couple of other disappointments, got me to a point where I wasn't willing to give new Paradox games a chance anymore. Fortunately, someone finally convinced me to give CKII a try, and I bought it on sale. I'm glad I did, but I'm also glad I didn't get it right away, because had I done so, I probably would have abandoned it after a couple of attempts, given what I've heard of its initial state.
There are still a few things I think the original did better than CKII--for example, whereas in CKII, sieges always progress at a set amount every 12 days, siege progress was somewhat random in the original--but overall CKII is a far better game.