All you need to do to enable it is type "setx /M MP_FORCE_USE_SANDBOX 1" without quotes at the administrative command prompt or powershell. It'll be made default soon though.
Remember, Windows Defender (Windows 10) Top Ranked By AV Test so it's not a horrid waste of space anymore.
Once that is better integrated into the lower level areas of Windows I expect it will be mainstreamed in maybe 12 months.
Windows 10 has a lot of other security improvements in it and users of Windows 7 and 8 should upgrade now.