If you are a gamer you may want more firepower, but for most of us, we don't need it. Most computers, even old ones, can do most of the things we use computers for: e-mail, word processing and using the Internet. Also processor performance increases haven't been huge in recent years...diminishing returns and all that. It used to be that CPU performance would leap drastically, but now that is no longer the case. Performance increases are actually quite small.
Microsoft wants everybody using Windows 10 so bad they are offering it as a free upgrade to all Windows 7 and 8 users. So, Windows 10 upgrading is really being pushed, but in order to run Windows 10, you need a PC or tablet with a lot of guts and older PC's may not be able to cut the mustard. That could mean if you want Windows 10 you may have to either upgrade or buy a new computer with Windows 10 already installed. If your computer will handle Windows 10 and you are using Windows 7 or 8, you get a free upgrade to Windows 10, but you have a year to do it and it's probably a good idea to wait and see what kinds of bug problems Windows 10 might have.
Windows 10 restores a nice feature of Windows 8, an operating system that was built more for touch screen stuff, plus it has a Windows 7-style Start menu and the ability to work on the desktop. I actually I do that with Windows 8 anyway.
Also, if you have Windows 7 the desktop gadgets don't work on Windows 10 and will be deleted when you upgrade.
Microsoft has a habit of trying to force us to use its latest operating system by ending support for its old operating systems, halting security updates and leaving us open to security threats, but if you have Windows 7 or 8, that's not likely to happen for years. Security patches for Windows 7 will be issued until January 2020 and for 8 until January 2023.
I don't use OneDrive in Windows 8.1, but for those that do, with Windows 10 OneDrive won't work because of the upgraded software for it.
Then there is the issue of things like printers and scanners. Microsoft doesn't always make sure that drivers for older devices work and some of your peripherals might not work. You don't want to replace a perfectly good printer, do you?
There is also Media Center, something else I don't use mostly because it's a nightmare to set up, but for those that do, guess what? Media Center won't work with Windows 10.
Another thing that irks me is Microsoft Office. When I bought my previous laptop they wanted another couple of hundred dollars to install Microsoft Office. I discovered that Open Office and now Libre Office. Both work just as good and are compatible with Office and they are free.
Even if you have an old, slow PC you can boost its speed, sometimes for free. Check out PC World's 10 cheap or free ways to make you PC run faster.
You can also upgrade to an SSD for under $100 depending on how much storage space you need.
One word of caution though if you are running Windows XP - that operating system was ditched a year ago so you should get rid of it for security reasons. How? Replace it with Linux.
Fortunately, when I purchased my new laptop a couple of years ago I was still working and getting rid of the pre-installed bloatware and setting it up was easy...I took it to work and the IT guy did it for me. All that crap is annoying and some dangerous, so it really needs to be removed but it can be a real hassle unless you are a computer nerd.
Then there is the cost. New PC's cost a considerable amount of money and so, ask yourself if you really need a new computer or would you rather have the extra money? Scammers are at work with Windows 10, too! Check these out…
Windows 10 users warned of two scams
From ZDNet..New Windows 10 scam
will encrypt your files for ransom
Scam Alert: Microsoft Is Not Upgrading Computers To Windows 10 Through Email Alerts