Things change. Once Sprint’s network is retired, other than US Cellular’s limited network territory, Fi’s multi-network flexibility evolves. By evolve, I mean that much of Sprint’s current network footprint but not Sprint’s legacy CDMA technology will ultimately be integrated into T-Mobile’s current network.
Beyond that unlike earlier GSM and CDMA technologies, 4G LTE and 5G will be compatible across all U.S. cellular networks making reciprocal roaming agreements possible if the various network operators so choose. In other words, evolving cellular network technologies make a Fi type approach much less necessary.