If Nest moved to a subscription model how do you propose they handle people who stop paying? Do they cut off remote access and write off the cost of the product as a loss?
They could follow the model of phone companies: they only 'give' you a phone if you sign up for a x year contract, may let you pay some sum up front, and cut off your use if you stop paying.
The economies change a lot when you can book that revenue up front. Many SaaS startups want to lock in year long contracts even if the total amount is less for this reason. That revenue can then be used to pay salaries, etc. Not everything has to be or should be paid by VC dollars.
Nest may have figured they could build a larger ecosystem in the IoT and Smart Home space and charge monthly at some later point but didn't materialize.
Those who can afford a Nest can also dump it if they choose not to pay. I love my Nest. Bought it one month after the launched. Would I pay $10.00 a month just to reulate my heat only? Hell no! There are so many new alternatives in this space - not as intelligent or sexy looking though. Maybe I build my own. The algorithm is not hard to implement. I am just lazy.
Still love my Nest. Sexy, smart, set-and-forget. But we only mingle in the winter.