A more direct answer to whether its problematic is "yes, but not because bonds are constantly maturing." The real issue is balancing the return and the duration of a bond portfolio, that is, the sensitvity of the portfolio to interest rate cahnges. The use of different maturities is necessary to acheive the desired yield of the portfolio while maintaining an palatable duration
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How Do Bond ETFs Track Their Indexes?
Traditional bond indexes make great benchmarks, but terrible portfolios. Most equity ETFs hold every security in their index. But that's usually not possible with bonds. Bond indexes often include hundreds, even thousands, of individual securities. Buying up all those bonds for an ETF's portfolio is not only difficult, but expensive. The cost to purchase thousands of bonds in illiquid markets, even those with minimal impact on the index, can quickly erode returns.
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