I think your misunderstanding.
This doesn't mean the free upgrade from 7 or 8.1 will be available for 2 to 4 years, its saying that rather than releasing a paid for 10.1 or 11 like they would have previously, they will be releasing updates over time. From an accounting perspective they have to define what that timescale is so that they can defer the revenue over that period, as per the legislation around accounts. In order to do that they need to define what they consider the lifespan of the product to be, which they have set at 2 to 4 years depending on customer type.
So what this means is that if you buy retail Windows 10 for $200 and they define consumer life as 4 years, Microsoft books will not show $200 of revenue in 2015, but will instead show $50 in 2015 and the rest deferred, which will be recognised as $50 in each year 2016, 2017 and 2018. You can't recognise revenue for a product or service until you have delivered it. This practice means that Microsoft will not have 'delivered' all of Windows 10 until 2018.
Its a shift in their accounting practices to reflect the shift in the product. What it does mean for us though is that the upgrades and improvements Microsoft are delivering for free will come for at least as long as they define the lifetime of the product to be.
