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

No, diamonds wear. It is not a myth, nor is it a conspiracy. To think that it is a myth is to have not have gone out to the barn and looked into the horses mouth.

Interesting, you know this for sure....and given my parameters posted above. Or, is this just supposition?? How have you 'looked into the horses mouth'? 

Years ago, I looked through a microscope of a stylus that was supposedly worn out, I had no idea as to whether the facets on the diamond were in fact correct or not...the dealer told me they were worn. 
What does a worn out diamond stylus look like? ( a shibata shape or a micro ridge or an elliptical?)  Are the diamonds that are attached to our cantilevers that accurately reproduced? Would like to know how they are produced with such accuracy... inquiring minds etc.,

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...)

Awhile back when my Clearaudio Gamma S had about 3000 hours on it I sent it to Germany and asked them to re-tip it as I assumed it was nearing the end.. They sent it back and told me that it was fine and to only worry about it when it started to mis-track. Considering that I had given them the go ahead to tip it and they did not and given that they have all the right equipment to check it, I can safely assume that they were correct. They did however ask what I was using to clean it as the tip was very gummed up. I had been using LAST. I switched to Clearaudio elixir as I figured that if they know how to make a $15,000.00 stylus, they must know how to clean it!

So.. I ended up selling my Gamma for a good amount of money considering it was used and bought a Clearaudio Victory H. I keep my records clean with a VPI, my tip with the Elixir and.. importantly, I think, I pay attention on how I drop the cartridge into the groove. I don't just drop the que but lower it gently into the lead in groove trying to avoid any sudden hard snap thus hoping to extend the life of the cantilever as well. I have had this cartridge  for about 5 yrs now with maybe 2000 hrs of use and have not heard any anomalies in playback as of yet. Letting your ears tell you what is going on with your cartridge may well be a good bet. 

One side of one average album is ~one mile distance.

According to the Wikipedia article about the LP record, 

The average LP has about 1,500 feet (460 m or about a third of a mile) of groove on each side.

So both sides of a record combine to make about 2/3 of a mile of grooviness.

I'd read that 1400-1500' figure other places as well.

Phono diamonds are man made, not the natural ones you find in most engagement rings.....  not nearly the same quality.  If indeed they are man made, can they really be called diamond...or diamond-like.

I can't tell if a diamond is worn using a 20X loupe. I was thinking of getting a 5 MP, 20-300X digital scope; but a totally gonzo cartridge/stylus photographer and re-builder on another forum posted that you need a combination of microscope and camera optics to get sufficient depth of field to see wear.

For $50 bucks, I may still try it if I get a new stylus for my MM. Then I'd compare old and new and see if I waited too long or long enough. Since the process is soooo gradual; I don't trust my aging ears.

I will say that I have a Shure V15RS with the original HE stylus from 20 years ago. I bought 2 new JICO styli, a SAS and an xMR. The HE and MR sound somewhat comparable, with the HE being stronger on the low end and the MR on the high end. Shape or wear??? I dunno; but I use the HE on older LPs with more surface noise. It is more forgiving and I figure the damage is done already.