My first 'real' headphones were MDR6s/V600s from Sony, maybe 20 years ago. I was DJing so went for closed back, and to me open headphones just seemed dumb. 90% of the time I was listening to music in public, going places, so I always went for maximum isolation.
I ended up with some Etymotic ER-4s, with the triple cone silicon tips. When wireless came around I got the Dash in-ears, and used triple flange silicon tips there too, so again, total isolation.
At home I used Sennheiser Amperior, also very closed.
Recently I felt the need for something very light but wanted to easily pop them on and off, so grabbed some PX-100-IIs, and holy shit.
The audio is fantastic, soundstage is so huge, and at the same time, being able to hear everything through them is great. I can have music on super-low and hear everything that's going on around me.
When I got the Dash, it was amazing to be able to turn on the microphone and hear the audio around you, but also have music on. It felt like AR for the ears. I had no idea that nice, light, open backed hps did the same thing already.
Yeah right? Just yesterday I got me the Beyerdynamic DT990Pro, my first open headphones as well and I LOVE IT! I had the same reaction like you had, open headphones are truly something else.