IMHO most analysts underestimated the enormous size of Windows 8 disaster.
Windows 8 user adoption rate is not only small, it is negative.
The sense of this statement is that machines does not live forever: there is an average lifespan for machines, and if the user adoption rate of the new system is lower than what is needed to replace retired machines it means your market share is reducing.
If a desktop PC average life is 5 to 10 years, 12 months/year, that means the replacement rate needs to be 1.66% to 0.84%/month
If you are the company owning the 90% of the market, like Microsoft, you need to keep up with the 90% of this replacement rate if you want to maintain the market share, say you need to get the new product to 1.5% to 0.75% of the market.
A rough approximation may be: if you are Microsoft you MUST sell to more than 1% of end users each month, or you are losing ground - that nowadays doesn't mean someone is necessarily buying a Mac (that was happened with Vista) or an Ubuntu box, but more probably people are moving to alternative markets (tablets) or moving data and programs to the cloud in the effort to need to use less PCs.
The problem with 8's user adoption is not only is smaller than Vista's one, the real problem is that it is well below the bar of doom - 1% - to point out loudly Microsoft is losing ground every day in its very stronghold.
Last months, despising the immense traction of Office launch, in fact it was below HALF of the death bar!
Even more worrying, is the fact Windows 8 aims to a wider market, so it would need to grow even faster to keep up with competitors.
Face the numbers of the disaster, Ballmer, Windows 8 is far below the death bar while being 1) actively promoted by the largest MS advertising campaign so far 2) being sold to a wider market than any of its predecessors 3) notwithstanding the costly launch of dedicated hardware and port to ARM world 4) notwithstanding 30% of your user base is on XP, good but 12 years old.
The latter part is especially worth of attention: 6.x kernel improved security and stability of NT.x kernel, but not dramatically; 7 and 8 improvements made the kernel quite a good performer, but the system is still huge and on low end hardware hardly matches XP, and on high end hardware it is more efficient, but again not in a dramatic way.
OK, XP was released in 2001, 8 in 2012, it contains 11 years of development from one of the biggest software house in the world.
Now compare what can you do on XP and 8, and compare what you can do with XP and a system 11 years older than XP, Windows 3.0.
And after that let's talk about MS' missed decade.