Why upgrade to Windows 10? I'll throw in my 2 cents even though I largely despise Windows as a result of supporting it day in and day out for the last 11+ years of my life.
1 - it's going to be supported much longer than Windows 7 or 8 are. This seems like a 'well duh' statement, but it's important to remember that Microsoft does not support their different operating systems forever. Windows XP already doesn't receive critical security updates, Windows 7 has gone past its mainstream support end of life (1/13/2015) and is currently on extended support. Granted that extended support is until 1/14/2020, it's going to run out and eventually the trickle of bug fixes and updates are going to slow from not only Microsoft, but from other software providers as well. Windows 8's extended end of life goes to something like 2023 which is a long way off, but if MS is willing to give you their next longterm OS now free for a limited time it makes sense to a point.
2 - they've overhauled the OS in a big way and reduced a lot of bloat. The same way that the Windows 7 out of the box seemed to run better than Windows Vista did out of the box (of course a dead cat ran better of the box than Vista). They've identified efficiencies in the code and how it runs on the hardware to take better advantage of what's already there while cleaning up the bugs that exist. We've seen it at work where the newer releases of the Windows 10 OS run better than the previous versions.
So no, it's not going to be a silver bullet that's going to allow you to magically run something in a much better way, but it can extend the life of what you have by enough of a margin and if you're going to be building on what you've already got regardless then it makes even more sense from the obsolescence standpoint.
Like I said, my 2 cents that everyone has probably already considered, but it's worth restating and rethinking.
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