TSP G Fund as the only Fixed Income Investment

Here’s something I’m debating and hoping the collective brain trust of the HD community can push me in the right direction.

As a retired Fed, I have access to the Thrift Saving Plan G Fund.  For those not familiar, it’s basically a high yield savings account on steroids.  It pays rates comparable to medium/long-term Treasuries but has no default risk, duration risk, or interest rate risk.  It only pays interest and can never fall in value.  There is no comparable investment available in the private sector.  The rate changes every month.  The current rate is 4.5%. All pretty good!  The Bogleheads often refer to it as the greatest free lunch in investing,

The downside “might” be that since the principal does not fluctuate, it is not subject to appreciation in a falling interest rate environment like one would experience in a traditional bond fund.  Also, since it’s based on Treasuries, the rate is going to be relatively conservative.

My question is this.  I’m considering using the G Fund as the only fixed income piece of my retirement portfolio and not worrying about a variety on bond funds, TIPS, laddering CDS/Treasuries etc.  Since it’s so secure, I could probably even drop my total fixed income $$$.

There is an issue w the TSP that I don’t like which is how they handle inheritances when the primary beneficiary passes away but I can prob handle that by leaving instructions for my surviving spouse to roll over any balance to a private IRA upon my untimely demise.

Anyone have any strong thoughts either yay or nay on this?  My initial thought are the upsides outweigh the downsides but maybe I’m missing something.   Thanks for your input.