In reality the 7X57 never caught on. No significant army ever used it and it's very hard to find ammo for it anywhere.
7X57 is still a major player in Europe and many other parts of the world, just not so much here in the USA where the 30-06 overshadows everything else. If you really have a hankering for a 7X57, you can buy a very fine rifle in that caliber from one of the European makers if you don't mind mortgaging your home for it.
Many of the South American armies used 7X57 for rifles and machine guns, and to dismiss them as not being "significant" is a disservice, as they fought one of the bloodiest and longest lasting wars on the planet. It was also used by the Boers (or Free Orange Republic), who managed to give the British a major headache for several years. The Spanish used 7X57 to fight their Civil War, and they were significant enough to attract the attention and respect of Joseph Stalin, Adolf Hitler, and FDR.
As for being hard to find ammo for it, all of the major manufacturers load it, you just have to go to a real gun store to find it.