My Refurbished MM(Late 2014), 2.8GHz, 8GB Ram,256 SSD is still on OS "Yosemite(10.10.5)" in which I bought in June, 2015. It has and still is a fast running machine that meets "most" of my Computer needs, however, I have installed a couple of "Steam" games that require a OS upgrade to "Sierra(10.12)". I have a concern that the Apple Folks will come-out immediately with another OS Upgrade at their WDC grand event next week in San Jose, CA. My thinking is that since OS "Sierra" has been in existence for quite some time in which Apple has weeded-out most of the "Bugs" that the time has come to NOW do this 10.12 upgrade. Any of your thoughts on this matter will be appreciated.