Earlier quoted context omitted.
You know the cool thing about this integration ? Any part can be replaced. Some instantly, some take more time because they involve physical stuff. People use the Google integration because it's there by default, not because they are locked-in.
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.