What made me change was a combination of things.
Plasma 5 finally becoming stable enough for me since 5.9. Me being fed up with inconsistent theming all over the place on gtk desktops was another reason. (okay, Gnome Shell excluded. Theming is pretty sweet there, but I can't stand the DE. I find it a horribly restrictive experience and not intuitive)
Another reason following that is gtk apps blend so well in Plasma that I can still use certain gtk apps I prefer or have no qt alternative for, like gufw or deja-dup.
In the end Plasma just gives me a good looking desktop which is now actually stable for me, without crashing. (until KDE decides on big changes and they fuck it all up again, as is the cycle. Why overdevelop something that functions well and offers tons of stuff? Aaargh!)
The amount of options and sometimes weird locations they are hid still drive me crazy sometimes, but I guess it isn't that much of an issue because I only have to set it up very rarely, so.
I think I like Cinnamon and Plasma equally at this point. Plasma just has better theming overall and since I can't handle visual inconsistency across a DE and it triggers my OCD quirks, Plasma wins.
Oh, and you're welcome (No idea where that actually was shot though):
I found it through duckduckgo search