I watch the podcast from TWIT their windows weekly program. This week they are focus on the new Window 10 release
The guests are long time follower of MS, so they kind know what they are talking about in regard to the Microsoft.
The indicated that:
Window 10 is free, if upgrade within 1 years of release from Win 7, 8, 8.1 it is free for the life of the device. That is what they said. Now, what does that mean?
For a PC, change MB and CPU will probably killed it. However, does change HD counted? How about Graphic card? We have no idea at this time. May be may be not.
But sure it is interested to see what is coming from the OS.
As for why should you move up to Win 10, you don't have. However, Win 7 is going to dropped sometime. All of the developers would eventual start to write software based on the Win 10 platform. If your plan is to use what you have for the rest of device time. Sure, you don't have to upgrade. But I think most of other simmer might have different idea.