I personally don't think it's fair to say that Google ran Nest into the ground, but here's my understanding of events.

Google acquires Nest, things are going ok, Nest hires a ton of people and Google is, at first, patient. Then years go by and Nest doesn't really put out any major products. Some people blame Google, some people blame Tony Fadell. Fadell is former Apple and basically set himself up as the Steve Jobs of IoT devices, so lots of projects getting killed for not living up to his standards.

Meanwhile, Google acquires Dropcam, which at the time had really started picking up steam in the IoT, and they do it without Fadell's input and throw it onto Nest. Fadell and Dropcam's CEO are polar opposites of one another and basically come to hate one another. Fadell starts shitting on Dropcam acquisition hires, and Dropcam's CEO starts publicly talking about how he regrets selling his company to Google/Nest.

A year later, Google restructures into Alphabet, and Nest is now its own company independent of Google. This posed two problems for Nest: Alphabet won't fund Nest to the same extent Google would, and Nest can't hide behind the massive advertising revenue that Google makes. Alphabet expects to see results now, not at some point in the future.

Fadell leaves, he's replaced by Marwan Fawaz, who came from the Motorola acquisition.

I'm against the opinion that Nest has been run into the ground, but Nest is supposed to be the key in Alphabet's IoT play, and it so clearly isn't anymore. Yeah, they make sensors and cameras still and the product line has finally filled out, and I'm sure that the Nest thermostat still sells well, but the biggest product in Alphabet's IoT future is at Google, and it's Google Home. Really, it should have been nest that lead the smarthome assistant, that operates as the hub for all your IoT devices, but really Nest has ceded that future Google, Apple, and Amazon. Now they're just another device maker playing in a larger ecosystem.