I do most of the work on my bike except for areas that require specialized equipment or tools that I don't have or can't afford.
Pretty much all engines these days are very well built.
If properly cared for, practically all of them will last a long, long time regardless if a twin or four cylinder.
Any engine abused or mistreated will prematurely wear out.
The high-mileage you speak is just a resale reference, just as for cars.
Beyond an arbitrary limit, the resale value falls.
That does not mean the machine is worn out. Unless it was abused and really IS worn out.
Unfortunately, the 600s are used as throw-away bikes due to their price.
Younger riders use 'em, abuse 'em, buy another.
The larger, more expensive bikes generally don't follow that pattern.