Firstly "lose" is passive. Google is ending support.
But most importantly "support" sounds like they just won't be fixing bugs and security problems. What is really happening is that Google is disabling all online functionality.
"Google is shutting down all online functionality for 1st and 2nd gen Nest thermostats in Oct 2025" would be a much clearer headline.
Indeed, had they started to support a product they would never say “15th gen Nest thermostats found support”. You know, they were walking through the woods and stumbled on some Google support. Isn’t that a nice coincidence.
I'm guessing since the generations being shut down are explicit, I'm assuming there are more generations that aren't being shut down, so the simple answer is that they want to sell more of those devices, basically force people to upgrade.
So indirectly, letting people still use the devices locally, would mean they'll sell less of the newer devices, which probably have better margins, slurp up more data and will eventually be shut down too.