The "best plan out there" ... with 2G ... LOL.
Presumably you mean Sprint (and T-Mobile). Yes, it is fast enough for many people. On my recent trip to Portugal, I did spring for 10 euros for eight days of fast data on my Dutch Vodafone SIM, but had it cost much more than that, I would have lived with 2G data just fine. I still come out way, way ahead using my Vodafone SIM when I travel vs. Google Fi.
"Expensive phones." Their low end phones have always been 200-250, except for a short period where they were out of stock. Wow, breathtakingly expensive. And with $100 trade-in value 2+ years later. What person who can afford to travel in Europe can possibly afford a $250 phone.
What person who can afford to travel in Europe can "afford" a $500 phone? Or a $1,000 phone? Probably just about everyone. Sorry, just don't get your point here. Just because I can "afford" something doesn't mean it's worth spending the money. I saved about $150 by not buying one of the Google Fi phones. I only bought the Moto E4 (a phone that serves my needs just fine) for the price because I got a free year of Sprint with it - well worth the cost. My phone service will cost far less than yours for a year. Saving money may not be important to you. Personal choice.
Some people complain about coverage in the US, see above. In my experience everywhere I have used it in the US it has worked just fine, as it rides on the Sprint, TMobile and US Cellular networks.
Sprint coverage has yet to be a problem for me. Coverage in rural areas is spotty, but so is T-Mobile's. If I really didn't care about the cost of service and needed reliable coverage even in obscure rural areas, I'd be with Verizon, not Google Fi or Sprint or T-Mobile.