I haven't been able to get my mind around bond index funds.
With stock index funds you get a % of each company in the index you are tracking equal (as best the fund can make it) to the % value each company has in the index. That means as companies do well and get bigger, you own an increasing amount of the company, and as they do poorer, you hold less. Seems good.
But what does a bond index track? Do they try to get a part of every bond issue out there? Do businesses issue bonds because they are going to grow and be more successful, or because they aren't doing well enough to raise the capital on their own? Do you really want to own more bonds from a company that has to borrow more and more money? Or is so much of it government bonds that the other bonds are just noise?
You raise some important questions. I'll not really try to fully answer, but give a bit of a different perspective to speak to some of what I think you are getting at. I think you have 2 broad areas: (1) Why do companies issue bonds? (2) How do index funds work?
I think funds mostly use statistical sampling techniques so that they don't need to try & buy some of every bond. There are fundamental difference in the nature of bonds & stocks that make this much easier on the bond side. There are a few factors that affect the price of a bond: credit quality, duration, etc. So, when a bond is bought, there is somewhat of a ceiling in place for that bond's return & certainly for bonds with the same characteristics. With stock, the investor has greater risk, but also a chance at upside making more of a differentiation.
Companies should issue bonds when that is cheapest form of capital & capital is "needed". Now some may issue bonds because they can. A few years ago Microsoft, Apple, etc were issuing bonds...because they were in trouble? or because capital was cheap (almost to point of being free)? Think thru the company accounting for debt vs equity & you'll see the impact is quite different from both the company side & investor side.
As to question of buying more bonds from a company that has to borrow more and more? Think of it this way...you may also own shares in that company on the stock side. If the company is borrowing inappropriately, there is likely more risk on the equity side than the bond side, right?
For the most part, the lower volatility of bonds lessens the distinction between specific assets. It is important to select the right area of the debt market. The individual bond owner or fund manager can scrape a little bit out on cap gain/loss, but coupon payment will account for lion share of return usually. If the index calls for "investment grade" & the company fails to sustain that level, the fund will have to sell that bond.
Always a good idea to not invest in things that we don't understand, although probably few really understand all the nuances.