I wouldn't say there was a definitive final straw. It was more of a death by a thousand cuts. Everything getting shittier and worse over time. Pixel phones originally were great, but I really wanted a great 1st party watch. Pixel Watch finally came out and it was lightyears behind the Apple Watch.
Software integration wise across the entire ecosystem as well. Google has been promising it, but Apple seems to have actually delivered it.
The Nest stuff hurts though. I've invested well over $3k in various Nest devices over the years, and to see what Google has done to the ecosystem is a shame.