I have a lot of friends who have tried to upgrade to Win10, and have found that even if they get it to install (and more often than not, it does not install smoothly), then their computer does not run well afterwards. If Win10 does not install perfectly, you may well have problems running other programs as well as MM9.
Besides, there have been all sorts of complaints about Win10 and the way it attempts to install itself when people don't want it to do so.
How MM9 runs may well depend on how well Win 10 installs. I decided that I'm not going to use Win10 (or its successor) until I can do it on a new computer.