Currently we are at the lower bound.
I've shown several areas where cost drivers are showing strong inflationary pressure.
China is tracking currently at close to 5% inflation, with food inflation over 10%. Many other areas of the world are also seeing strong price inflation, especially related to food.
So, while the cost of capital is currently near zero, this is not permanently feasible....and we roll over debt..so there will be a time, in near future where we are forced to refinance the massive debt you are asking to be put in play at 5% instead of 1%....and if it gets worse, we have times in our recent past (last 30 yrs) where that cost of capital has been 12-15%.
You are correct in saying that we are not Ireland. However, we are getting very close to carrying a similar debt load...and a 4 or 5 point financing penalty on that level of debt is no longer an annoyance...it quickly becomes a disaster.
The key term in your "why" question is "substantial". A couple of points on the debt load we're carrying is substantial...it doesn't have to be a "Weimar-esque" hyperinflation to be a real problem.