My friend has a 5K screen and it is lovely. Trouble is, I've got years of photos I've resized to fit my 1920x1200 24" Dell monitor screen. I can see hours of work ahead of me! Good thing I'm retired...
I don't know what you do with all your photos, but if you just want to occasionally view them without the hassle of resizing, you may be able to (temporarily) reset your monitor's resolution to 1920x1200: System Preferences>Displays>Display tab>Resolution scaled