It's an Internet connected computer that has a temperature sensor and relays. Computers run operating systems, and those operating systems require constant updates to patch vulnerabilities. When those updates stop, the clock starts ticking on when they'll become attack vectors. You don't allow attack vectors to access your servers.

The only thing being taken offline is access to their servers (which is a plus for me). The thermostats still function as thermostats.

So no, it's not a fucking thermostat. If you want one that'll last 50 years, go buy an old mercury thermostat or one that relies on other laws of physics instead of literal computers. Everything has an expected lifespan.

Honest to God, I could have sworn I remembered Google bricking these same devices like 10 years ago, which is why I find it weird that anyone cares about Nest products. I built my own smart thermostat (super easy, you just need homeassistant, an ESP32 or pi pico, a 4x relay board, and a sht-3x sensor (plus 18vac to 3.3vdc or 5vdc converter to power everything). The hardest part is an enclosure, but I guarantee there's a nerd like me in your city that would design you one for fun (literally, building custom smart devices is what I do for fun)