I "upgraded" to FF - long-term (and very happy) 7D Mk II user, I decided to treat myself to a 1-Dx.
And much as I appreciate the 1-Dx (especially in low light - the extra stop or so over the 10,000 ISO practical upper limit for me with the 7D Mk II, has proven very useful) I really miss the resolution and effective reach of the 7D Mk II, so I still want a high pixel density crop-sensor upgrade too.
But...
If an FF camera was introduced that had a high enough pixel density to equate to a pixel density upgrade on a crop (without compromising on frame rate etc), that would do for me. But I'm in a position where I'm not overly constrained financially, so this might be a non-starter for others, because it would not be a cheap camera.
The point being that for me and people like me (I won't be alone in my preferences) there still isn't a crop-to-FF upgrade path that would satisfy what I want from such an upgrade.