When it comes to sharpening stones, diamonds are certainly not forever. They have an initial period when they cut very aggressively and then level off for quite a long period, but they absolutely do wear out. How quickly, does, of course, depend on how much you use then and for what. Very hard steels will wear them more quickly. I don't use them for chisels and plane irons because I find they don't give a fine enough edge - but I admit I am a bit obsessive about sharpness. For this, I use Japanese water stones whch give me what I want and don't blunt but they do wear down and need flattening, for which I use a 120 grit DMT continuous surface diamond plate and in this role, they last a very long time. I do have a fine DMT which I've had for many years but I only use this for re-sharpeneing things like Stanley knife blades and secateurs.
I have a couple of plane irons which are super-hard and very little will touch them. For those I use diamond pastes on Corian blanks (the plain stuff not the type with stone inclusions) which is very effective; the diamond seems to embed in the surface and is renewed as necessary with fresh paste.
Jim