I can't quite answer your question as I haven't, and hopefully never will, give up on 3D. But I have spent a long time comparing 4K and 3D, so here's my 2p worth.
I was spurred into buying my 4K television, when it became obvious that television manufacturers were going to drop 3D. From everything I had read, passive 3D on a 4K television was the best 3D possible experience.
Now owning a 4K television, I was very keen to try 4K and for me the experience has been mixed to say the least. Most of the 4K films I've watched have been a step up from the Blu-ray versions, but not a huge step up. Some have been a worthwhile improvement others are just so so. And a few have been worse than their Blu-ray counterparts - at least in certain scenes.
For me the joy of Blu-ray 3D is that it offers something new - spatial information. 3D is quite unlike DVD and Blu-ray. We perceive the real world in 3D, just as we see the world in colour. For me, 3D makes the experience, more realistic, more immersive and more enjoyable.
What does 4K give you? More of the same, but better, but nothing new. Colours are more accurate, there is a little more detail in the picture, we have better specular highlights. These are all nice to have, but they come at a terrible cost. The complete and total loss of all that wonderful spatial resolution.
When I watch a film in 3D I'm drawn into the experience. I never sit there, thinking. I wish this film had better colour or more detail. I don't miss those improvements at all.
When I watch 4K, the experience is much the same as watching Blu-ray with a couple of incremental improvements.
Initially, I used to buy both the 3D version and the 4K version of films. After about six films, I stopped because once I played both versions to compare them, I knew I'd never play the 4K version again. I would always choose the 3D version. There simply isn't a comparison. The 3D version has always been the far better experience. It's not even close.
If there are lots of films on 4K that you want to upgrade from 2D Blu-ray, then by all means go for 4K, as long as you realise that many films are just a marginal improvement.
But as far as I'm concerned, 4K isn't an upgrade over 3D , it is a downgrade. A very, very big downgrade at that.
Of course, 4K is the newest shiniest toy AV toy around, and for some that is reason enough to buy it. And that's fair enough, but for me, I want the most immersive, realistic and enjoyable film experience I can get. And that is 3D.
I should add, that I don't only watch 3D on my 4K TV. I have a cheap 3D projector, a budget 3D LED TV and a head mounted 3D display and 3D is wonderful on all of them. Even my cheapo 3D TV offers a far more enjoyable experience than watching 4K on my OLED set.
In short, for me 3D is wow! and 4K (at its best) is 'nice'.