IMO, Microsoft is primarily a software company. They make money when they sell software. Having Windows 10 run on old computers = more software sales for them. So it's in their best interest to have Windows 10 run on as many legacy devices as it possibly can.It feeds into one of the positive aspects of Windows 10 over Mac OSX.
Continuing to use older hardware is better for the environment then 1. Using energy and no being able to fully recycle every component out of a laptop then 2. using energy to make a new laptop.
I applaud Microsoft anyway for giving better legacy support than Apple - it keeps computers usable. Goodness knows a 2008 White Macbook on Lion doesn't get any security updates anymore or anything. Pretty poor imho. Not to mention how well Windows 10 runs on the 2008 White Macbook - app loading etc is almost on par with my Mid 2012 Macbook Pro with an SSD + El Capitan. Yet Apple is meant to have this great 'hardware/software' integration that no one else can match...
Microsoft is doing this to make money. Any impact it has on the environment/energy is purely happenstance.