Those guys were not ballbashers -
And Sampras and to lesser extent Olderer have big serves to back up any risky play. Thus they can get away with more risk even when they are off. Del Potro has no similar serve to back up his bashing. His movement is also poor compared to aforementioned players. Sampras played on faster courts where a lot of balls simply didn't come back. Del Potro plays against greatest defenders ever in Murray/Djoker/Nadal - Olderer's defense isn't too shabby either and his has the serve.
woah woah, we are not comparing Del Potro to tennis Legends. All those guys you've mentioned will surely end with a much better career than Delpo's (with the exception of Murray, but he's pretty ahead career-wise anyway).
What Delpo fans mostly argue about is that Delpo is a heck of a dangerous player, has 2 big weapons and an overall solid build, and is capable of winning slams.
There may be overall better players out there, but this guy right here has awesome potential.
He also offers (along with Federer) great offensive tennis, which is great for the game to counter the boring time-wasting monotony of the so-called 'greatest defenders ever'.