Any gun that fires when you need it to is relevant. Everything beyond that is just shades of applicability, which vary wildly based on the circumstances and the shooter.
That being said, I think the Hi-Power is an excellent gun whose prevalence worldwide dwarfs even the acceptance of the 1911 in these United States. Just as the 1911 represents the apex of 45 caliber handgun design, the Hi-Power is the ideal platform for the 9mm Luger round: It is slim and light and takes advantage of the compact round by providing amazing capacity without sacrificing grip ergonomics. The Hi-Power is the oldest 9mm handgun still in modern production, and almost all modern semi-automatic handguns sprang from its design. Its numerous progeny notwithstanding, the Hi-Power can still be found in its original form for the same reasons as the 1911, and for the same reason that sharks and crocodiles are still successful predators despite the fact that their forms have remained virtually untouched by evolution for hundreds of millions of years: Sometimes, things are done right the first time.
This isn't to say that the Hi-Power is perfect; like any work of man, it has shortcomings. None of these drawbacks are unintentional, though; all are design tradeoffs or intentionally implemented. The strength of the gun, though, lies in the fact that it is the product of an earlier handgun design mindset that remains relevant despite attempts to brand it as pure anachronism. The Hi-Power, like the 1911, is a handgun designed for one purpose: to shoot well. With a singular shark-like focus, all elements irrelevant to this purpose, unless imposed by external fiat, have been stripped away. What remains is nearly the Platonic ideal of a semi-automatic pistol: flat, compact, ergonomic, accurate, reliable, and quick to recover from recoil. Safety mechanisms exist to prevent inadvertent fumbles, but keep well clear of the primal interface between the shooter and the gun. Compare this to modern safety mechanisms, many of which act to prevent the user from firing the gun easily. By contrast, the Hi-Power acts like an extension of one's hand, melding seamlessly into the intermediate space between the shooter and the target.
Art is what remains when all that is unnecessary is removed. In this sense, the Hi-Power is a work of art. Like any work of art, its status as such is debatable, but one thing can be said for sure: It's still relevant and useful to those who like its ergonomics and shootability, and it will continue to serve a valuable role long after many of its supposed successors have drifted into obscurity.