Can you share any reference on what the Nest servers actually do that a smartphone chipset isn't capable of? Various apps manage (from my limited knowledge about the field) quite impressive things.
Can't really discuss specifics, but training machine learning models that share data between all your cameras would be pretty difficult, for example. Also, the lifespan of the processor would be highly reduced if you were constantly hammering it (thermal implications, etc. etc.) Again, in a few years that might not be an issue anymore :)