AFAIU, the Social Security Trust Fund will stop soaking up deficits some time in 2019 as it transitions to a net outflow of cash. I can't find the numbers but presumably this means (and has meant for several years, now) that the public (non-intragovernmental) share of yearly deficit funding has been steadily growing and will really take off next year. That in turns means foreign buyers will have increasingly more leverage to effect interest rates.
Forget about the share of outstanding debt. That's money under the bridge. Think about the year-over-year change in the share of deficit spending funded by foreign buyers. Considering how explosive deficit spending will be over the next decade because of the tax cuts, compounded by higher interest rates, we could be in for a very bumpy ride.
What once were irrelevant swings in interest rates, current account deficits, domestic savings rates, etc, could quickly come to dominate the trajectory of the economy.
Sure, the Federal Reserve could soak up the deficit. But that would be much more likely lead to inflation than in previous periods.[1] Higher inflation means foreign buyers pull out, which leads to greater inflation and less capacity to purchase the imported goods that we're completely dependent on. It also means less profit on the international market for the high-margin services and industrial products we specialize in. Europe is doing a much better job than the U.S. at controlling deficit spending, and the Euro could easily become the new reserve currency.
[1] And don't forget, the Federal Reserve's mandate is low inflation and low unemployment. Congress may need to step in to change their mandate to de-prioritize low inflation, which would send a very nasty message to investors. Decades of easy money because of our current account deficit (aka trade imbalance) and strong domestic savings (via Social Security) has left people with a twisted perception of the consequences of budget deficits and inflationary monetary policy.