I'm watching the outcome of this with guarded enthusiasm. Nest being spun back out as it's own company under Alphabet is good to me. I disliked their integration into a business who's primary focus is advertising.
There is likely many reasons this was done, and a good chance part of it is shady business reasons. One that I haven't seen discussed much is the change in liability. I think Google learned some hard lessons when they bought Motorola for the patents. Had the patent lawsuits all went in favor of those fighting Google, it could have put far more then Android at risk.
There is still a large liability target with Google today, but it's at least aligned with all the business units focused on ads. Nest's liability if someone dies from a faulty Protect, or the liability with the self driving cars is now isolated to those businesses. At least I believe so, but I'm not fully up to speed on the laws in this regard with the type of reorganization they are doing.