Because Autodesk switched to subscription-only models, you can legally purchase a "perpetual license" transfer for 2015 versions from a reseller. However, note that you will still face the obsolescence issues listed above.
Q: What are the system requirements for Xf-adsk2015-x64? A: The minimum system requirements for Xf-adsk2015-x64 include a 64-bit Windows operating system, 2.5 GHz or faster processor, 8 GB or more RAM, and 10 GB or more free disk space. Download Xf-adsk2015-x64