In my view Fidelity and Schwab are better than Vanguard on customer service and website, but equal or better in costs.
I believe the way Fidelity Zero funds make money is with block trades of institutional investors. If you sell $1,000 worth of stock, the market absorbs it. But if someone dumps $10 million of mid-cap stock, the price can fall as they sell. This "market impact" incentivizes institutional investors to find another way of selling. So they reach out to a large mutual fund, and pay to sell their $10 million in stock. The institutional investor pays less in fees than they would in "market impact" costs, and Fidelity Zero can use the money to pay for expenses. They could be other things going on as well, but I thought an example of how this could actually work may help.
One downside is the Fidelity Zero funds were only created 4 years ago, in 2018. But it's been a dramatic 4 years, so not a bad test of their model.
Personally I prefer ETFs, so I'd rather VTI or ITOT than stick with a Fidelity mutual fund. The 0.03% expense ratio is $3/year on $10,000 invested. That's unlikely to impact retirement plans.