Somebody in Redmond must have looked at the uptake rates, and the falloff percentages over time, and done a calculation, and realized that they aren't going to make their "numbers" (One Billion Installs by some time in the next few years) unless they start forcing it down people's throats.
Its not like they didnt have this anyway with corporations.
They are contractually in an upgrade path, for corporate licenses. So this is really aiming at the individuals and smaller shops that are not locked into forced upgrades due to licensing contracts.
Actually, they say that their number one support question is “How do I upgrade to Windows 10?”