won't be able to purchase a new machine with Windows 7 or 8.1 on it. If you computer has Windows 7 or 8.1 it will get updates until the support end dates. You just won't be able to get a new PC with Windows 7 or 8.1
This is how it happens. Microsoft has always supported it's operating systems with current hardware updates till the time the software was to have it's support expire.
Now a new policy of non support for new hardware for W7 and W8 after mid 2017 will be followed in increasing effort to move more over to W10 quicker. What this means is if you build or buy a computer after that date, and it's hardware is newer than that date, your W7 and W8 will not work properly with the hardware on those motherboards, unless the motherboard manufacturer has developed and supplied it's own driver files for every chip on the motherboard. So, many new motherboards by end of 2017 may no longer be able to run W7 and W8, even though the latter is supported till 2020.
"July 17th, 2017 is the killer. This is the last possible date Microsoft will allow Windows 7 or Windows 8 to be used on a cutting edge computer and still receive essential support like security patches. If this sounds a bit odd that’s understandable because Windows 7 and Windows 8 were meant to be supported until 14 January 2020 and 10 January 2023 respectively. But under the new rules that only applies if your hardware stays in the dark ages. "
Of course you can move to W10, or prefer a Linux distro (which will continue supporting new hardware till it's LTS support cycle ends).