There's a part that already can't be replaced while keeping all the others. Google Fi only works with Nexus devices. It could be a lack of OEM interest, but who knows what Google does that might keep it that way.
Google Fi is an MVNO, one that entered into an area with arguably insufficient competition. Regardless of how you might rate the level of competition before Google Fi entered the market, it has definitely added ambitious customer-oriented features.
Its device-dependence exists because it's a unique dual-provider MVNO (Sprint and T-Mobile). This allows them to broker the best deal for the end consumer. But the bar is high -- devices must support dual-SIM, which is rare. Devices without explicit support aren't barred from Google Fi but may not get the full features and you can't get help with those devices if you run into issues.
