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.
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.