Realistically I think the two sweet spots are 120hz and 240hz - not necessarily because they are the best of the best but because they are each divisible by both 24 and 30 (the most common FPS of films and television) AND they offer two tiers of increased performance for different hardware requirements. You can run a much more taxing game at 120 and then if you want to spend the big bucks on the latest hardware move up to 240.
As for resolution I completely agree with you - 1440P is really a sweet spot for 27" monitors. If display / DPI scaling improves across multiple OS then I think eventually we will likely have 4K become the norm for 27" sized monitors and it will show some improvement but again be diminishing returns like the difference between 120-240. That being said as more film content moves to 4k I think we will also start to see 1440P become less popular as people will want to view content in something that doesnt scale.
All of this however is nothing compared to the improvement that a true HDR display brings - a high end monitor that can show a large increase in dynamic range is such a game changer and I do not think most people realize it yet - it brings us so much closer to how the human eye really sees that I really think it is equivalent to the difference of going from laserdisc resolution to something 4k. And on top of that now that cameras are also shooting in such massive dynamic ranges it is going to make older content just look plain in comparison.