When the currency is a commodity that is traded on an exchange, that takes a lot of control away from the government. I feel that the international trade and exchange value of the currency is the more dominant factor in the operations of the monetary system. You really need to read my work it provides a full explanation of how currency operates and the tools governments can use to influence it. I feel you have over valued credit and ignored equity which can derive from domestic or international sources. I also feel you have not appreciated there is a difference between credit and bonds. Credit operates differently to bonds and is more frequently unsecured, bond repayment schedules are different and they have a maturity payment. Credit is more expensive, usually higher risk and requires a much tighter repayment criteria. If feel it is more bonds you are trying to explain and they have a different nature to credit. On another note how would MMT in practice deal with high bond maturity payments? You know when governments have to get massive amounts of money in a short period of time to buy out their massive mountain of debt that has a maturity date that needs to be paid off. Are they just going to QE a few $Trillion in one go and then not expect that to have an impact on the currency or economy. Wow! maybe credit is better for MMT, but it comes at a much higher interest rate, yes that is right you have missed the whole Sovereign debt crisis repayment burden of bond maturity payments on what $20 Trillion of government debt. So that's not going to cause a problem if you just QE massive sums like that to roll the government debt repayments over when the maturity date hits? This is the reality of government operations when they have massive public debt. They can't pay it off in one go or even in small instalments and most of the time the debt is growing. They have these massive bond issuances that have huge maturity payments and either have borrow more to cover it or enter into a sovereign debt restructuring mechanism. You say just QE, Great Hyper inflation!
Central banks are why MMT does not do a great job of explaining exchange rates. A theory is only as good as it is at predicting the movement of spot rates, and no one uses MMT to run forecasts. What you extrapolate given a theory of MMT, that a gov can run deficits as long as it issues debt in its own currency, is not because of MMT, but because of central banks.
Treasuries roll coupons all the time. Treasuries call or buy back higher yielding debt and issue new lower yielding debt when the interest rate environment allows for it. It is a core tenet of their operational mission. When the US sold very high yielding 30 Year Bonds and 10 Year Notes in the 70's and early 80's I guarantee you they didn't stay in circulation for long. That's how Japan has managed to carry such a high debt-to-GDP ratio for 30 years now. Taking advantage of low to no interest rate environments. And Japan's Central Bank (BoJ) buys half of the debt issued by Japan's Treasury so it props up demand on the secondary dealer market. I'm not saying that it's the right thing to do, but that's how Japan has managed to date.
If you trade enough interest rates you learn that the cheapest-to-deliver is usually the most "callable" or desirable to buy back by the Treasury in terms of cash flows (clean price).