TSR, good questions. If you like the simplicity and peace of mind of all-in-one target date funds, then I wouldn't lose too much sleep over the I Fund's deficiencies (I Fund == MSCI EAFE developed markets index fund). Yes, there are many threads here (and I've written a few) talking about how to put together a portfolio without the I Fund. But what's interesting is if you compare the EAFE index returns with Vanguard's Total Stock Market Index Fund, and the Fidelity FSGDX, you see the returns are not that far off:
The chart above goes back to 1996. Note that the Vanguard Total International Stock Index has changed composition over the years.
line is an ETF (ACWX) that tracks the same index as Fidelity FSGDX (I chose this ETF because it has an earlier inception date than the Fidelity fund). You can see there's not much difference between any of these and the MSCI EAFE index (orange
Where these lines get more interesting is when you look at specific funds for the asset classes that the MSCI EAFE index omits (emerging markets and small-cap stocks):
line is the Vanguard emerging markets index fund (VEIEX) and the red
line is the Vanguard International Small Cap index fund (VFSVX / VSS). As you can see these two funds are quite a bit more volatile than the broader index funds. But they're not big enough to move the needle much with the Fidelity FSGDX or Vanguard Total International funds because they make up a small portion of these bigger indexes. (Emerging markets is 17% of VG TISM; small-cap is ~10% of VG TISM.)
This is similar to comparing returns of Vanguard's Total Stock Market Index fund with a 500-stock index fund -- not a huge difference since they have ~80% overlap. But if you look specifically at a small-cap US stock fund, you'll see much more variation.
Put another way, if you want to juice returns beyond VG TISM or TSP's I Fund, then you probably need to overweight one of these asset classes (small, value, emerging markets) by a pretty big factor.
All of which is to say, I wouldn't stress too much over the I Fund. All that being said, if you ever wind up splitting out your TSP funds for whatever reason, then you might as well leave the I Fund out of it, and perhaps boost G Fund since your other accounts will of course not have the G Fund if you want all/some of your bond allocation in the G Fund.
Disclaimer: Past performance is no guarantee of future results, etc. etc.