That's just it. You don't know what will happen 50 years later. You have to assist your allies in the present, you don't have the luxury of cutting them off on the assumption that they will be your enemy via a coup or revolution or whatever a half-century later.
Maybe England and Taiwan will be our mortal enemies in 2056. But should we treat them as such now?
May be your mortal enemies are in your country. May be USA will collapse just as USSR. Have seen some information, it will happen in 10-15 years.