If you're with Fidelity, their Spartan fund series are the ones that will match up with the Vanguard index funds.
And it's easy to recreate a target date fund blend using 2-3 of these funds. It just means you might want to check them once a year to see if you need to manually rebalance. There is a school of thought (which I agree with) that you don't need specific international funds if you're in the U.S. stock market, since many many companies that are U.S. based have international exposure.
And it is my understanding that as folks grow older, they are wanting a smoother ride instead of the short-term (1-5 years is considered short term) volatility that comes with being weighted heavy in the stocks, so that's when you start adding in bonds - to smooth out the yo-yo effect. But if your portfolio can weather short term drops, then staying a bit more weighted than usual as you age can mean a higher rate of return... but again, you have to be prepared to have some lean years in there if your portfolio itself is just "adequate" and not "robust."
https://www.fidelity.com/mutual-funds/index-funds/why-fidelityI have three funds:
FSTVX - total stock market index fund - 0.06% ER
FSITX - total bond market index fund - 0.17% ER
FSRVX - REIT index fund - 0.19% ER (this is not really necessary if you want a simple stock/bond portfolio)