There are a few enthusiasts here who have in excess of 10,000 miles on their favorite rides and many bikes are also more than 35-40 years old and still in daily service. Your bike doesn't look worn so much as probably neglected and somewhat abused. Go to China and see how many POS are pressed into daily service.

If there is nothing but surface rust, I wouldn't worry. On some bikes, the brazing sometimes breaks down, but that is the exception rather than the rule.