And yet as someone who has used Samsung's Touchwiz and stock andriod, I don't think they are different enough to care therefore instead of a custom OEM ROM i want stock andriod so I can at least get security updates and OS upgrades.
The hardware manufacturers are fooling themselves if they think their custom OS is a differentiator compared to their competitors.
For YOU maybe that is the case, but if focus groups of normal people complained about Touchwiz it would be gone. Samsung and all these OEMs who are successful have skins for a reason.
Honestly stock Android is pretty rough sometimes. We like it but we are nerds who are willing to try new buttons or swipe a notification shade down twice to get at toggles. Samsung has an Easy Mode for a reason, a lot of people can't handle that stuff. They get scared or confused when it isn't obvious.
Plus when you consider Touchwiz has features like multi-window it becomes clear that part of why OEMs make skins is to keep a better parity with Apple on features. Google doesn't sell phones. I mean not really, Nexuses have a small marketshare. So they aren't motivated to match that iOS feature or make this or that task super easy for mom. Google can't provide a total solution for everyone. They make Nexuses for themselves.
Finally most regular users don't like updates. Updates can change things (bad), cause the device to run slower (bad), or simply require enough space to install them (which requires deleting some cat pictures aka bad). Users who aren't us don't like OS updates, they are just a necessary evil in technology from their perspective. Android as a business model has been sold on a lie that updates are not important and I think a lot of users like that lie.
Quite frankly Android is almost a mature market. If fast updates and stock Android brought in a lot of customers companies would do it. But we have seen the Google Play Edition program fail, and Lenovo is pulling back on having that be a Moto trait. We only represent ourselves.
Just be happy the Nexus 6P is such a nice phone.