None of the major carriers can even scratch that price point for a single device. Most are upwards of twice the price per month, plus BS activation fees etc.
I've considered switching to other MVNOs like Mint, but when I take a look at the pricing, it seems like the only way to go lower in price than what I currently pay is promotional rates. And if the price will spike after a year, I'm not interested in going through the process of switching my line over, creating a new account, and all that jazz. Fi works well and costs me a reasonable price per month so I stick with it.
Another benefit: if I ever get a cellular smart watch or a cellular tablet, you can get data-only SIMs on Fi for no monthly cost -- it just shares the same data pool as your phone. That means I could someday run a laptop, hotspot, phone, watch, tablet, etc. all off a "1 person" Fi unlimited plan for $70/mo all-in. Not too shabby.
Which prepaids? The last time I compared Fi to my prepaid plan they were roughly the same cost.
I understand that "unlimited" plans mean I'll generally eventually hit a limit where my speeds get throttled but $2/GB is not comparable to Google Fi at all.
Obviously, for some people, they won't care about these features/downsides and consider the plan worth it. But comparing it to Google Fi as an equivalent but cheaper option is just misleading.