The ever lasting diamond stylus...is it a myth or??

I have had some MC cartridges sound "like New" even after 10 or 15 years of play.

Some die after a year.

Suspensions go, long before the diamond, IMO.

I've seen diamond tip worn out to the base of the cantilever.
They wear out just like pensil

One side of one average album is ~one mile distance.  Yes, diamond is harder than vinyl, but if you have gunk in the grooves; like dust and dirt it will wear down (grind) the diamond over time.  We are taking 3000 - 5,000 miles use.  The record groove will also wear down.  The first 'loss' you will notice is at the high end; treble.  Don't forget the groove is shaped like a 'V' and at a specific angle.  The sound waves are pressed along the side walls of the 'V' almost to the bottom.  The diamond will travel along the sides of the 'V' until it hits bottom or the sound grooves are worn away.  Good record cleaning and stylus cleaning procedures are mandatory!  

You hit upon an important item.  Many different diamond grinds exist for a stylus. One type may last longer than another, but one may capture the sound waves better than another.  I do not know which shape is the 'best'.  

My Lyra has a 'line contact' diamond.  It is manufactured by a company in Japan that makes the pure Tungsten needles for scanning electron microscopes.  I believe a company that can make devices for a SEM certainly could handle attaching a diamond to a single Pure Boron Whisker.   It's been my experience (over 40 years) that I become bored of a cartridge prior to it ever wearing out.......

There is absolutely no doubt that if the groove is very dirty and has hard deposits...that through time the diamond can become a little worn or possibly damaged.( as to the shape). However, as in my OP, I am thinking that today there really is no need to have a dirty and dusty LP. Plus there are several lubricants that can be applied both to the LP and to the diamond stylus to reduce friction,,,,,,leading to my OP.
czarivey......a ’diamond tip worn out to the base of the cantilever, they just wear like a pensil ( sic) ’ I really would like to know what kind of use this cartridge was put to????

They don't last indefinitely but, yes, way longer than people think if you keep your records clean. The rubber on the cantilever will perish before your stylus. (Five years or so, although a couple years back I used a vintage Stanton from the eighties that honestly sounded great so, who knows...)