The only thing that would impact federal spending in a meaningful way is some change to SS and Medicare/Medicaid
Social Security + Medicare + Medicaid + Net Interest = approximately 60% of all federal spending, leaving 40% for everything else, including defense, veterans benefits, transportation, education, law enforcement, homeland security, scientific research, foreign aid, and other federal programs. There is no political willpower to make any changes. SS likely won't go broke, but with enough interest rate pressure, our politicians could be forced in to reconsidering benefit structure. Foreign Treasure holdings decline could have a notable impact on interest rates. China isn't exactly "dumping" our Treasury's but they hold about 48% of what they did in 2013. They can't very well just dump it all or it would hurt them as well...