Google Fi is offering $100 to upgrade your 3G phone before it stops working - The Verge

Google Fi will stop activating non-VoLTE phones in January, and encourage customers with older phones to upgrade with a $100 credit provided they purchase a new phone from the Fi store and activate it before December 31st, 9to5Google reports.

Since T-Mobile has already announced its own plans to phase out support for non-VoLTE phones starting in January 2021, Fi has to as well.

Upgrading to better cell service is a long term good for the usability of phones, but it is an inconvenience to customers.