You're right - they were just overviews of the previews of M. I did notice that many things were changing in them too - I found it odd that the way pages were laid out was so fluid/changeable compared to the previous M preview given that UI guidelines should dictate how things are laid out.
The fact that things move around so much would be indicative of not being really sure how it should look (despite the UI guidelines), perhaps? If I was buying a car and the next version of the car the next year moved the dials to the other side of the dashboard or swapped whether the indicator stalk was on the right or the left of the steering wheel, you'd ask what the heck was going on. Yet this appears to be normal for Android.
I would love to blame my phone manufacturer for not getting updates, apart from Android was started with the Open Handset Alliance. It doesn't appear to be much of an alliance to me.
It's a daft situation. If you bought a laptop and could only install hotfixes and KB releases via Dell, yet Dell did not give them to you, would you be happy with Microsoft? Would you take their advice to just buy a Surface Pro from them? That would be unrealistic because it would ignore the massive market that is Wintel devices; rather you would expect Microsoft to only provide Windows to Dell if they agreed to provide hotfixes Microsoft gave them.
If you had a MacBook Pro and El Capitan would not run on it, yet you were aware of all the security holes in Mavericks and some in Yosemite, would you accept advice to just buy a new MacBook to get security fixes? Or would you expect Apple to backport fixes when they know that a sizeable percentage of customers are connecting to their App Store with older versions, given your purchase fee of the hardware?
The solution to not getting updates for a device is not to buy a new device.
Google boasts how many devices they have all over the world (see last year's io keynote) and how Android is everywhere, yet it failed to mention the important point about Android being everywhere: that it is insecure outdated Android that is everywhere.
Google know how many people are connecting with older versions (the Play dashboard tells you what percentage is running which version), so they should push out updates or mandate in their agreement with phone manufacturers that a timely update cycle should be followed, or the agreement would be terminated. There would be no other way to keep control or maintain a high standard.
I say this with a plethora of Android devices and with my dayjob of writing Android apps. It is a rubbish situation for security.