Well…
I’m a bit in the right hand tail on this subject (see my rig), but for me the 43" 4K monitor I added to my battle-station was worth it. Beyond game design and 4k video, what are your use cases?
I’m a VERY visual person. I like to have all my research (including multiple instances of my course text open to different sections) all over my screen. For this, the big center monitor is invaluable.
Again, more use cases would help. Also, 1 4K could replace x3 1080’s in productivity. I’ve been busy with work and school, so I haven’t had much opportunity to game on my 43" 4K. I suspect it will be fine. I play Blizzard games. I also purchased Stellaris - that might be wild on a big monitor. One use case that I am looking forward to is multi-boxing. 
Is it worth - only you can answer that.