Does not “safe(ish) activities such as holding government bonds” contain the fattest most dangerous tail risks?

Sir, Gillian Tett writes “the Fed and the Office of the Comptroller of the Currency introduced proposals to “tailor leverage ratio requirements to the business activities and risk profiles of the largest domestic firms”. In plain English, this means banks can operate with a little less capital to absorb losses, provided they focus on safe(ish) activities such as holding government bonds.” “Trump’s mixed record on rolling back bank reform” April 13.

The Fed’s Governor Laid Brainard, in a recent speech “An Update on the Federal Reserve's Financial Stability Agenda”said: “The primary focus of financial stability policy is tail risk (outcomes that are unlikely but severely damaging) as opposed to the modal outlook (the most likely path of the economy).”

So let me ask: What is the tail risk of “safe(ish) activities” compared to that of riskier activities?

How fat or dangerous is the tail risk of what is rated below BB-? Very skinny indeed.

How fat or dangerous is the tail risk of what is rated AAA? Very, very fat indeed.

Government bonds? When in 1988 the regulators, with Basel I, decided to assign a 0% risk-weight to some sovereigns they painted themselves into a corner. If that risk weight is not increased, then sovereigns will become, sooner or later over-indebted, and their risk will grow until it hits 100%. If that risk weight is increased, ever so slightly, markets will be very scared. How to get out of that corner is the most difficult challenge central banks and bank regulators face. Let us not forget that in 1988 US debt that was $2.6 trillion. Now it is US$21 trillion, growing, and still 0% risk weighted.

PS. The only way to solve the 0% sovereign risk weight conundrum that I see, is to increase the leverage ratio applicable to all assets, until that level where the risk weighted capital requirement totally loses its significance.

@PerKurowski