Let's first set the inflation issue aside: if that worries you as a foreign buyer, then just buy TIPS since it accrues at headline CPI (not core CPI).
Now, let's talk about your core argument. While it's not an unreasonable one to make, I disagree: In the case of the US, China as well as all export intensive countries are essentially the same as domestic savers. China incurs dollar denominated reserve because of its trade surplus with the US. That can either sit at 0% nominal yield (which is a negative real yield) in cash, for in treasury bills/notes at above 0% nominal and quite possibly non-negative real yield. If the choice is between 0% yield and a positive yield, then clearly China, like the Japanese domestic savers, would take the positive yield however small.
For all the talk about reserve diversification, China cannot fully meaningfully divest its dollar reserve without causing a worsening terms of trade that hurts its exports. China's domestic demand is unlikely to replace the foreign demand for its goods in the short to intermediary terms. In order to continue its growth (on which the legitimacy of the Party is predicated), it will buy dollar assets with its dollar reserve.
Unless there is a radical change in the way central banks operate, the bulk of those dollar reserves will be in treasuries (with some agencies; though if take downs by foreign banks in the few recent FNMA and FHLMC auctions are any indication, not too much more agencies debts).
So, it appears to me that the dollar, due its status as the reserve currency and the currency in which a vast amount of demands for goods is denominated, attracts a global poor of 'savers' who have no choice (and indeed is in their interest, much as the JPY intervention has displayed) but to buy treasuries at whatever the yield.
Quote from benwm:
If US inflation on most assets is 3%+ (irrespective of the fact this might be conveniently ignored by the Fed due to understated 'core' CPI i.e. excluding food, energy, VAT etc - note CPI excluding everything is zero!),
then a carry return of 2.5% less Fed Funds is not so great. It is not comparable to a Japanese domestic borrowing from the BoJ and earning a similar return in JGBs.
Especially if you are not US based and are exposed to the risks of holding USD, an important consideration for (foreign) buyers of USTs, less so the risks of holding JPY for (domestic) buyers of JGBs.