Nokia's demise

Their mistake was relying on an outdated platform too long (Symbian) and then not being able to develop or acquire a successor platform that would keep them in business as a totally independent phone manufacturer. Whatever they tried with Microsoft was doomed to fail from the start.
AFAIK, that's Microsoft's usual strategy. It rarely keeps acquisitions as stand-alone entities. MS usually scavenges out the parts they think they want, and scuttles the rest. Most usually, this is in software company acquisitions, but here in hardware as well. So, some parts of the Nokia acquisition will go on, it's just that the heredity will not any longer be known.

And yes - the acquired company's employees are usually in the soup line, excepting a few that are cherry picked off the top.
I'm still using my Symbian Belle phone.

It isn't so bad, consider how well it works with a 650mhz cpu and 512 ram against similar or higher specs Android phones. They were modernizing it when Microsoft acquired them and announced the all their new phones will use Windows Phone which from start to finish never had all the features of Symbian, a loyal customer who wanted something new wouldn't find it with them.

At one point they had three systems Windwos Phone, Symbian and S40 on Asha, thats a bad idea.

I think the reason they failed was the lack of apps, with Windows phone they failed to attract developers, for Symbian it was HARD to develop, the Qt SDK for Symbian came to late, and Asha phones were very limited low cost phones.

True. I still have my Nokia E63 from early 2009, works like a champ, built like a bullet. Limited functionality but what it did, did it greatly.
Before acquiring an Android phone(I changed 4 in last 2 years), I bought a Lumia 730. Fantastic hardware as expected from Nokia, but pathetic Windows 8.1 app collection. It looked like made by kids for kids. I couldn't find a simple note management app which could read and manage my collection of notes. (Forget one-note, it was too complicated for a phone)